Okay I may not give the right answer but it's my opinion and I tried. If that is the case maybe you should ease off on having sex. I think it's normal for cramping to happen. It's best to make a call to you mid-wife or nurse just to be on the safe side. It's also normal to not want to have sex early in pregnancy I was the same way too but after your second trimester you'll start getting back to your usual self.

Hey jess, cramping is normal for most people, just if it gets bad you're probably gonna have to lay off the old for a while.

Even before I got pregnant sometimes I would get a stomachache afterwards. We only just started back doing it recently (i just didn't feel like it)
all I can suggest is make sure you take it easy - hard to explain but relax into an orgasm rather than tensing into it, that way instead of a hard orgasm you will get a more soft body tingle.
If you aren't getting that far, amybe a different position would be better, maybe his willy is prodding into your cervix or the angle of penetration is causing pressure on your uterus, maybe try laying on your side with him behind you (spoons) or similar and preferably where the penetration won't be too deep (ok some like it deep and hard but most babies prefer it if you go a bit gentle)

good luck, take care