I remember feeling what you are during weeks 14-18 periodically. It freaked me out since I had a miscarriage at 15 weeks (with no symptoms at all!) but, I called my doctor and they ordered an ultrasound to show me all was well with our little one.
Like Sweetangel7 said, if you have severe cramping or begin spotting... obviously that wouldn't be normal and you would need to go to the hospital or doctor. But, sounds like what you are experiencing is pretty "normal" (what is normal, right?)
It can be very normal...as long as you are not experiencing bleeding, I would relax...your uterus is doing a lot of stretching and growing so that is probably what you are feeling.