It sounds like Braxton Hicks contractions to me. I'm 26 weeks and having the same thing. It makes me anxious sometimes and that speeds up my heart rate and makes me nauseous. I don't know if its the anxiety of it or what but the same thing happens to me. My doctor said if I vomit or feel faint to go to the ER but otherwise they are harmless. She said my uterus is "practicing" lol. By all means make sure you mention it to your doc the next time you go in, or just call them...I have called mine in the middle of the night...they are used to it:) Hope I eased your mind some. Good luck:)
I am 28 weeks and I can now feel my baby's hard butt and back sometimes when he is stretching/moving, but it doesn't give me nausea.
i would talk about it with your dr . stomach hardening usually is braxon hicks