I didn't have to stop taking it, it resolved on its own. I didn't actually have any bruising or bleeding though, it was just something that was picked up in a routine blood test. Normal levels of platelets are 140,000-440,000, I was at 134,000, so only a little below normal. My doctor didn't seem too worried and told me we wouldn't change anything unless it continued to fall. A month later I was 144,000 and a month after that 170,000 so it seems I'm in the clear. My husband has UC, he also had a period of low platelets while on asacol, and it also resolved on its own without stopping the medication.
Definitely see the haematologist though, but I wouldn't stop the medication until you know what's going on.