They say that sex is safe during pregnancy but in your third trimester, you may want to take it slow.
Yes that's normal. I'm 32 weeks pregnant with my second baby and the same thing happens to me every time my husband and I have sex. I've even felt like the baby was going to fall out! I talked to my Dr about it multiple times and she says it's completely normal. I even had some spotting recently which they say is normal too.
Orgasms do cause uterine contractions. If it continues or gets stronger it you start bleeding a lot, you should call your doctor or L&D. Otherwise some stomach cramping is normal. Evena little bleeding (spotting) is normal.