Orgasm cramps with IUD

I don't know if this is a me thing, or if this is common?

I got my IUD 4 months ago. When I orgasm, with my partner during sex, I don't get a cramp like pain on the right side near my hip.

But, when I masturbated with a vibrator I get the cramp like pain. It's not a extreme pain, since I continue till I finish. But it's enough for me to take notice.

I had this happen once or twice, through the same style of masturbating, quite a while back before the IUD. But it went away.

When I do my sting check, I can still feel it a little. So I dont think it's out of place.

Thanks!