I'm 6 months as well, and will once in a while get some bad cramping if I move wrong or get out of the car to fast, and also have mild cramping when I lay down. I think you just have to be the judge of what is considered really bad cramping. If it is physically hurting you, I would defenitely call your doctor. It is most likely just your stomach and uterus growing in size.
first of all, if its concerning you it will concern your doctor. go in for a check up, and be as detailed as possible when you describe to him/her exactly what's going on. TRUST ME!! I work for an OB, the best judge of something not being right is mom. Even if it turns out to be nothing, peace of mind is much better.