I've recently had some bad luck with antibiotics, and I had similar reactions to yours when I took Sulfa Trimeth. They started after a day of taking them, and then lasted until the day after I stopped taking them (I was on a 3-day course, so they were pretty potent). I had no appetite and bad abdominal pain that entire time, but I tried eating anyway even though it hurt.
I got bad diarrhea for two days after I stopped taking the pills, which I attributed to the killing of the "good bacteria." Then I was fine. Give your body a couple days to get back to normal, eat yogurt, and if it doesn't clear up see your doctor.