I have that problem with my husband. When they're well endowed and are going in too deeply, they can bump an ovary or nudge the uterus, which, of course, is connected with lots of nerves there in the lower abdomen. All that anatomy is very close together in there and there are lots of pelvic nerves that support it. The pain'll almost knock you into next week. Seems to happen most often in the missionary position for me.

The solution that works for me is to change positions so I'm better able to control the depth of the thrusts or adjust slightly backwards and away from the thrust as he's moving in. The woman-on-top position helps with this. On-the-side positions help, too. Doggie style works. So do positions where you keep your legs more together than open. That helps keep the deep plunges more shallow. You can also always ask your partner not to thrust so deeply, but when things are hot and they're getting close, I'm convinced that's not always easy for them to consciously control. Good luck!