A.P.
I'm going to third the food intolerance. Wheat and/or milk is probably the issue. An intolerance won't show up on allergy testing, so the only way you're going to know is if you just eliminate them for a while and see what happens. While medications may help alleviate the symptoms, they are not addressing the underlying problem. Also consider other food additives as a culprit too (preservatives and food dye are big ones).
About 80% of the body's ability to fight off illness is in the gut. Probiotics are really important to putting things back in balance after antibiotics.
Good luck and I hope you find a solution soon!