Rock Hard Stomach

Many times a day, I will notice my stomach get as hard as a rock. It will stay that firm for 10-20 minutes without stopping at all. Then it will soften and stay soft for at least another 15-20 minutes. Sometimes there are a few hours in between. This started a month and a half ago. I initially thought it was Braxton Hicks, but I've been told it isn't due to how long it lasts. What could this be?