Bumping into her cervix?

I doubt youre hitting her cervix. Its position prevents that. You might be touching her vaginal wall or some other part of the vagina. The vagina is amazing, its meant to handle any size penis without "running" into anything serious. Now that I think about it, you could be directly on her g spot which is the most pleasurable spot for a lot of women. If fully stimulated it can cause female ejaculation.