Trying to figure out what happened to me during my run today

Us flatlanders might blame altitude, but you should be used to it  

How was the humidity?  What did you do yesterday?  Is it possible you're getting a lung infection or something (unlikely, but we won't know without asking)? 

Fatigue and dehydration are leading candidates, as well as nutrition.  Maybe you just ate something, but if you skipped dinner last night (or dinner was a pretzel and beer), your body hasn't started to really digetst what you just ate. 

Pay attention to your workouts over the next week, and if it happens again, get to a doctor. 

My first thought was nutrition or dehydration. You said you ate, but how much then and how much throughout the day?

I have to agree with everyone else... nutrition.  I had a similar issue a month or so ago.  I got up and ran early one morning, which is unusual for me because I typically workout in the evening.  I didn't know what to eat, or how much really, so I ate a granola bar and drank some water then headed out.  I planned to run 5 or so and about died by mile 3 so I had to make myself quit.  Nothing else was different, so I had to chalk it up to nutrition and hydration.  Haven't had an issue since. 

I don't see how nutrition could possibly be a factor after only 2.5 miles.

it was extemely humid this morning...especially for Denver.  I didn't run but was out on my bike and I noticed everyone I came across that was running looked absolutely miserable! Here in Colorado we aren't accustomed to humidity but we have been getting more than our share the past couple months.  As the other poster mentioned, nutrition at 2 1/2 miles should not be an issue.  I wouldn't worry to much about it...I had a similar experience on a humid morning a couple weeks ago.  Next time you run you mihjt want to bring some water with you...even if it is only 6-miles.