unexplained weight gain

Are you eating clean?  Good, quality proteins, carbs, etc?  I would assume you are, but you didn't mention it.

Are you drinking alcohol?  Beer?  Just because I find that when I indulge with a few beers over a few weeks, I pop an extra 1-2 pounds on, really quickly.

Then, maybe it's muscle too...

Keep working out hard!

Not sure about your weight gain, but I would assume that the "flab" you're talking is result of losing muscle mass on your abdomen and having looser skin around your waist.  I know that when you increase muscle mass, you "feel" like you lost a lot since the loose skin is stretched over areas especially the stomach.

It takes about 2500 Calories to maintain your weight, so I would have to assume you are either being stressed out or taking in Calories that you don't recognize.  I would think that eating healthy also adds to that too.

Almost every tracker overstates the cals burnt by activity - sometimes by x2 or even more! You do not burn 1200 cals bike riding at 14mph for an hour. You do not burn 2400 doing a HM in 1:45. Etc

Rules I've found that seem to be pretty accurate. 

~20 cals per km on the bike IF you are working HARD. If you have a PM/Garmin, the cals burnt is pretty dead on. If you are just loafing along, you get almost 0 (because you are not supporting your bodyweight, unlike walking)

~For running (any speed), 1 cal per kg per km. If you weight 60kg, you get 60cal/km.

~75-100 cals per 400m swimming IF working hard.

The rest I just use a comparison to an hour of hard running - ie, if 800 for an hour of running, I'm not going to get 600 by an hour of bowling, regardless of what the tracker says. Maybe 150. Or doing an hour of crossfit/cardio with lots of rest/standing around, maybe 400. Etc

Also don't forget to count any gels or gatorade used during the workout. (Don't cut these to try and diet, cut the junk elsewhere)

It may relate to when you eat what.

What does the 2000-2400 cal consist of?

It's highly improbably that you gained 7 pounds of anything but excess water weight in a two week period.  To gain one pound of weight, you would need to consume an excess of 3500 calories.  Do you think you overate 21,000+ calories in 14 days?  Highly unlikely...

How 'wrecked' does your body feel?  If your training volume is really high, it's possible that the stresses on your body are resulting in your body holding onto fluids in order to aid in repair to any muscle tissue, etc.  Also...as a female, I tend to gain 5 or 6 pounds right before I start my cycle. 

I find that massage and self myofascial release on a foam roller helps aid my muscle recovery and reduces discomfort and water retention I may have from training.

Best answer so far.

I'm reading all the responses and I second this as well. Just for an example, after I finished my half ironman I went from 178 at the start line to 187 in about 4 days due to my body holding fluids (it took about 20 days before the water weight started leaving. I don't think I've peed so much in my life). So unless you build up to the extremely high training loads really slowly your body will retain fluids during periods of high volume training.

Because you're working out so much you're eating a lot more carbs, too, and carbs help your body retain a lot of water. I'm not going to say cut out carbs but if you're disciplined enough, try counting macros and see what that tells you. I'm no nutritionist so I don't know if telling you to cut back on carbs and increase fats & proteins to lose water weight will aid or hinder your performance, but it might shed some water weight. I know I can lose 4 pounds in almost 48 hours if I cut out (MOST) my carbs and replace them with lean meat like tuna and have a lot more veggies than I normally do. But then again I'm also a 180lb male so weight fluctuates much more for me than it would for you. especially with a massive sweat rate and working out a lot.

Also when you start tapering it might come off pretty quickly too. Not definite, but might.

As weird as it might seem, if your body is retaining water (and this is likely the problem), drink more water

Thanks for all the replies!

I agree that gaining 7 lbs over two weeks can hardly be the result of overeating that many calories..It's got to be the carb consumption. I eat pretty clean, but admit that the excess "cardio" of s/b/r has me craving sugar like crazy and carbs in general and I've been eating more of those than I would normally.  (pasta, bagels, etc).  I'm going to go back to my "usual" eating of fruits, veggies, and lean proteins and see if that doesn't get me back.  It was just freaking me out that the weight came on so quickly. In the past, I was training hard, eating more carbs, and losing (or at least maintaining).  So I expected to continue along that path with eating more carbs this time, too.  Guess not.  I do have a Garmin 910 and average about 17  mph on the bike, so I am pushing it, at least for me.  (one previous poster mentioned calories burned being inaccurate).  My last HIM I left town weighing 120 and came back weighing 117.  And I'm actually training harder for this one, so it was a surprise to see the weight piling on instead of staying the same or dropping.  It's got to be the carbs and water.  Thanks.

X2

Just two other things to consider-pregnancy and thyroid. Most women obviously have a good sense of if they're pregnant. I'll defer to you on this one. Hypothyroidism would be possible cause of gradual weight gain, but probably not what you're describing here. Could account for energy levels being relatively down (aside from just a lot of hours of SBR).  

I agree with the discussions above regarding carb intake and food quality...

Sadly this is true for many women.