I am pretty sure what you are feeling is the muscles in your stomach from tensing and relaxing. It’s just like exercise…if you do it right. It still happens to me on occasion.
What Sueanne says is probably true. Most likely you are tensing the lower abdominal muscles (i.e., giving them a work-out), and, you are sore afterwards just like you would be if you did pushups (and then your chest or shoulders would be sore). Some women do experience painful intercourse, which is known as dyspareunia. If that is what you are experiencing, you should consult your doctor.
My theory is this: there are two types of muscles…regular muscles and sex muscles.
So unless you have a really strange regiment at the gym,
chances are your sex muscles are just a little out of shape.
Well, that or you’ve added a new position or angle to your repertoire.
I know my recommendation may sound dire,
but it would seem the only way to improve on your predicament
is to have more sex. Lots of sex. Lots of really passionate sex.
It could be uterine related. Talk to your doctor about it. If you have (or if there is a family history of) uterine fibroids, the pain can continue to get worse until sex isn’t so much fun anymore. (That is when you’ll be wishing for smaller packages, if you know what I mean.) There are things that can be done, but it’s worth a discussion with your doc the next time you go in.
Before I had a hysterectomy, I had a retroverted uterus (meaning it tilted backwards), and it would actually get hit during intercourse, causing pain. Just one more thing it might be :)
My doc said “it’s just another way to be.” My aunt had the same thing. It was just always that way. Because my cervix was basically above my uterus, it made pelvic exams (especially during labor) quite a chore!
If it’s a dull muscle pain, it might just be that your stomach needs some more working out! Try more sex.
If it’s sharper than that, it might be an ulcer that’s being triggered by the stress of sex, and you should see a doctor and drink milk, and avoid coffee and citric juices (and other acidic things).