I have been suffering from quite a bit of pain in my right buttock since last friday after receiving an im injection there.

Possibilities: Any time the skin is penetrated, there is a chance that infection can develop. We are trained to use care to minimize this risk, but it can still occur sometimes. Assuming the injection was properly placed (the upper-outer quadrant of the buttock) the remaining possibilities include (1) local irritation from the medication itself and (2) infection. Heat and redness indicate infection.