Youre putting pressure on a nerve with the fluid. You can also hit a nerve with your needle which is 100x worse than the feeling you are getting now. When you hit it with a needle it feels like you have electricity flowing through that body part. Its quite painful.
Yeah, to me that is the most logical explaination. Sometimes you can even get that feeling the next day if you get a lot of swelling in the area like if you did test prop or some crappy test 400mg/cc stuff.