mine is a slightly different case i recently realized my stomach turned hard but is hard all the time not sure what it is just went to my doc and he's sending me for more tests
I had this same problem, I just saw the docted yesterday.
I also had nausea on a regular basis.
He said the hardening of the stomach was simply gas, and he gave me some pills to take to reduce the nausea and bloating.
A gastroenterologist would be a good place to start, or just see your family doctor first.