Yep. Nerves are a f'ing bitch. I bet its in your quad, right? More nerves in the quads than anywhere else. Once you find the sweet spot, it'll be easier. The muscle spasms because you're hitting the nerve. Back out 5/6 of the way, change angle, and try again.
I have the same problem. Use to, 1 out of 4 of my quad shots would hit a nerve. Talk about injection anxiety! It sucks, but its just another part of the game!
Chris