Contractions After sex

I'm 33 weeks today and last week I started having contractions after sex. It's definitely not cramping. My belly gets hard and then will release and its painful. It lasts about an hour. Sometimes it's directly afterward sometimes it will be 4 hours later when it starts. My husband mentioned that maybe it's time to stop which isn't like him. Unlike me he hasn't lost interest in sex during my pregnancy. But the contractions scare him. He had one baby born early and spend time in the NICU and doesnt want to risk that happening again. Has anybody else have this happen? Is it normal? Are there other positions that don't cause this to happen?