knee/leg pain....advice?

Yesterday I went for a longish run (about 8 miles) and around mile 7 I noticed a weird feeling behind my knee. It wasn't really a sharp pain but kinda just felt a bit tight. It almost felt like I spilled some sticky on the back of my leg and as I was running I was sticking together. Anyhow I finished the run and today the muscle in the back of my leg directly behind my knee cap is sore (almost the center of my knee behind the knee cap to the upper portion of my knee cap). It gets really tight when I sit so that when I first get up to walk I almost feel like I am going to fall over. The pain is not to intense but I am worried about what might be going on. I jsut got over persistent shin splits (thanks chi running) and really don't want another injury!! Any ideas what might be going on and advice on how to make it not happen anymore!!

thanks in advance!
Deanna

ps this is the first time I have had a pain like this....