Antibiotics can cause you to have abnormal bowel movements--namely diarrhea. If your stools have changed since you started on the antibiotics, I wouldn't worry; they should go back to whatever is fairly normal for you when you stop the antibiotics. Taking probiotics right now and for three weeks after you stop the antibiotics may help get you back to normal faster. Some people just discharge fluid and/or mucus with bowel movements or gas. The wettness in your underwear may be from that discharge when you pass gas. Again, if this is new, it could well be the antibiotics causing it, and hopefully it will go away again when you're done with them.
And no, your being able to eat all of that stuff has no bearing on your IBS diagnosis. Milk doesn't bother me. Krispy Kremes don't bother me. Spicy food doesn't bother me. Hot dogs are one of the most tolerable things I can eat, actually. And fries have never bothered me. Caffeine, on the other hand, tears me up. And fried foods, like Kentucky Fried Chicken, bothers me as well. So not everyone has the same triggers, although spicy and fatty are fairly common triggers. Also, people's IBS can go into remission: so what bothers you today, might not bother you tomorrow, but will bother you again the day after that. It used to be that I could tolerate a coke somedays, but other days I couldn't; I finally got tired of never knowing how they would affect me, so I stopped drinking caffeine all together.