Yeah well, UG's are notorius from painful Primo. Primo hurts when Schering makes it in a perfect environment. I remember the first UG Primo was by Supra and it was so ridiculously painful that no one would buy it. And AS users will put up with a lot of pain as a rule. Is it a knot about the size of a quarter? Or red and warm?
It could that you ruptured a vein putting it in as well it could be an infection but that takes longer to surface.