IBS-like Symptoms

Hi all-
This past summer I visited Egypt for two weeks. During this time I had a few instances of diarrhea, which I attributed to the stress of flying, the intense heat (120 degrees some days!) and the abrupt change to egyptian food. However, the day after I returned to the US I got a bad case of Traveler's diarrhea, where I couldn't leave the house, was horribly dehydrated, etc. I took immodium and 4 days of Cipro, but I never visited a doctor.
About a month later my girlfriend came down with what I thought was the stomach flu, anyways, she had diarrhea as well, and the next morning I had it again too. Nowhere near as bad as the post- Egypt case though.
Anyways, ever since then I've had a lot of loose/yellowish stools, occasional diarrhea, more stomach pain than normal. Sometimes I'll go for two weeks or more without these problems, but they always seem to pop up (often when I'm stressed or anxious or have to travel).
I've never had these kinds of problems before and I'm getting extremely frustrated. A visit to the doctor yielded nothing. I was tested for giardia and it came back negative, however I'm not sure what else it could be. I'm a fairly healthy college-aged person, I exercise, I don't eat very much Sugar / junk food, etc. I've been taking probiotics but these haven't seemed to help much. I also take a calcium supplement.

Suggestions? Help?