If they put you on probiotics at the same time, it might have been that. Took me weeks to figure out that it wasn’t the meds making me sick, it was the probiotics they recommended to keep the meds from damaging my gut. I just didn’t tolerate them well for some reason.
Last I checked, the literature was somewhat equivocal about the benefits of "probiotics in a pill" anyway, compared to something like a tablespoon of fresh lactofermented sauerkraut twice a day. That conclusion matches my personal experience too.
The literature is actually quite robust on the benefits of probiotics. Take a look at the different peer-reviewed studies just on this list of products sold in the U.S. [1]. The quality of these studies is quite high, and highly scrutinized.