Diagnosed with uc. had 3 remicade (infliximab) infusions so far. developed peri-rectal abscess.is this because i had chron's all along or side effect of remicade (infliximab)?

Both possibilities: Certainly chronic disease can cause perirectal abscess. We also know the Remicade (infliximab) increases risk for infection. Without knowing everything about your case i would suspect you always had crohn's. Your physician should be able to help you clarify that question.

Hard to say: But could be related to either, see our GI doctor and discuss.