Are you sure you are not having contractions? Is your stomach tightening up then releasing? My sister had severe "gas pains" as she called them until she realized they were contractions.
Can you call your doc or get to the hospital? That might be the best place for you. I had agonizing pains at 32 weeks that were actually contractions. The docs stopped the labor and gave me the corticosteriods to make my baby's lungs develop. They also gave me endocet, fluids, and finally morphine. This combo worked to stop the contractions, but only after 36 hours of hard labor. After three days I was sent home on bed rest.
It may be nothing, but you should really get it checked out. Don't play with pain in your final trimester, it may indicate something serious. Like they said to me at the labor ward "If you have doubts it is better to come down and get it checked out. We like it when you visit!"
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