It's called lightening crotch. I've had it since 37 weeks and it can knock the breath out of me sometimes it can be quite painful.
I'm only 15 weeks and I feel that sometimes too. I agree with the comment above mine lol. Probably just the baby deciding to sit on an ever so unfortunate spot.
Well it hurts lol and is annoying but thank god it's not all the time lol
Probably your baby pressing hard on your cervix or a nerve