It might be coincidence or it might be the vitamin. Centrum have that heavy coat, similar to asacol, and I didn't want to put any more stress on my colon, so I switched to gummy adult vitamins. You might consider giving that a try.
I also take iron as a separate pill (also a hard coated thing), Slow FE, which for sure I think is irritating but since I tend toward anemic, it's catch 22. If you took it separately like that, you might be able to just take it a day or two and not have it irritating every day.