Had similar experience. Had allergic reaction to penicillin first prescription, which triggered my full body to come out in itchy hives. Went through 3 courses of different antibiotics until I was fit enough to get out of bed. Had bad guts for a couple of years following, which was usually an alternation between diarrhea and constipation. Sometimes I'd sit on the throne several times a day and no movement, except mucus or eventually, blood, as I'd developed piles.
I went on a super high fibre diet, ate as much low calorie food as I could a day to keep things moving. Ate lots of fermented foods, mainly dairy, sauerkraut, cured meats. Eliminated sugar, took up cycling to work to get fit. I've not been ill since, my bowels are moving, and piles are gone. Not the most pleasant story, but thought I'd share it.