Tina Otte answers:
What you are describing sounds like the slight contractions of the uterus that are known as Braxton Hicks contractions. Your uterus contracts continuously during pregnancy, but you are not usually able to feel them until the second half of your pregnancy.
The release of oxytocin during lovemaking can cause these gentle contractions, and in some cases when you become dehydrated or you overwork yourself, or are distressed emotionally, they may be more intense due to the release of stress hormones.
If these tightenings become uncomfortable, and you would describe them as painful- please talk to your caregiver. Be sure to take things easy, drink plenty of fluids and rest whenever you get the opportunity. If there is any fluid or blood loss from the vagina when you feel these contractions get help immediately.