Have you tried going in at a 45 degree angle instead of 90? When I do my arms, I have to go in at a 45 degree angle. That is the only way I can get them done. You may want to give that a try and see if it is any better.

As for the pain/spasms - this happens to me every time I shoot in my legs. I stopped shooting in my legs and haven't injected in them for the past 2 years.