Hi Judy
First, not all antibiotics produce the same side effects in us. Some constipated me, some have zero digestive effect, an others give me diarrhea. My reactions to each don't always align with those of others.
2nd, I assume that you used Imodium or UK equivalent, an that directions were followed, i.e., start with 2 w/first occurrence of the runs each day, followed by one tab same day every time you're off to the races. I mention this because here in the States at least half of people using it never read instructions and start with one ,,, we are an illiterate people lol.
3d prebiotics, here are the dirty secrets: there are many different ones (as oppised to different brands & different deliverybfirmats) and they affect different things inside of you. Then there's the whole question of whether the ones we take are alive/viable. Further dirty secret: they don't work on the entire digestive system; other bugs in other areas need ,,,
,,,PREbiotics! Quinoa, psyllium fiber and inulin are all prebiotics. Oats may be another, and if you do some research you may find that there are many more. If I have to take one of the fiber drink mixes, I prefer insulin over psyllium but that's mainly because in college I got amoebic dysentery (kayaking on Lake Powell); in law school I had diarrhea for months at a time, due to endometriosis, so I've consumed so much psyllium ,,, Imodium hit US market as Rx med in my last year of the latter. Look them up & try them out.
I do find that good unsweetened Bulgarian organic local yogurt helps, plus kombuchas and prebiotics. But you have to stop the diarrhea ad much as possible an then work on re-establishment of healthy gut critters.
This is all a bit over-simplified and someone here probably knows much more than I do. Hope this helps a bit, and goid luck on the z-pack.