You can test if it is caffeine for you by trying sodas, chocolate, teas, and even caffeine pills.

At least in my own anecdata of IBS, caffeine alone does not have a laxative-like effect for me. I have noticed some but not all strongly bitter foods do, and heard a theory once that some flavoring compound that tastes bitter may be subtly enough like gall passes through the stomach to confuse the gut into thinking there may be a gall leak and to flush the system faster. That certainly sounds right to my anecdata, but also part of why I think I crave bitter foods sometimes (as an IBS-C to IBS-D switch/trigger).