belly growth in 7th month has stopped though baby is bigger than should be.

I just wanted to see what is going on with my pregancy! My doctor has discovered that at week 28 my womb as stopped growing. I went to an ultrasound to check baby's growth and the baby is showing as 1 week bigger than it is supposed to be. I am now just hitting 33 weeks and my womb is still 28cms. I am extremely uncomfortable in my back and ribs. The baby and myself are extremely healthy and have been, and still are, textbook cases aside from this enigma. Also, I have gained 30lbs since i got prego but have no fat anywhere on my belly and am mostly muscle everywhere else. My doctor doesn't understand this either. She isn't concerned but I was wondering what is going on and if this is normal. Also, I am wondering what might happen if I do not pop out soon, my insides can't handle much more crowding!