Hi michelle,
Gee i wish my advice was more constuctive, i di have this happen to me from about 30 weeks but did have a very large baby. I would definetley give your ob a call,just to see what he thinks. maybe if the placenta was sitting high you may get this feeling. Is it only like this at certain times,ie standing or laying?I found swimming or gently floating to feel absolutely wonderful as i got larger as it took the pressure off everything.