A.V.
I would try to not pay attention because the more you react the more she will do it. My car seat has a large padded circle that protects the child's legs and tummy from being pinched in the buckles, I would try padding the strap maybe the change would throw her off, and maybe it would prevent her for making herself bleed. I would also talk to my Dr for ideas I know it makes you feel embarrassed but it is really best if you can learn how to be matter of fact about sexuality and not ashamed.
Best of luck.
A.