What are these?

Last week I was sitting down and the left side of my belly got super hard, it didn't cramp or hurt so cant be contractions or braxton hicks, I don't think? It last for about 15 seconds. Then last night I woke up to the same thing. The left side of my belly super hard but didn't hurt at all. It lasted about 30 seconds or 40 seconds. What could this be? Im 25 weeks, should I be feeling this at 25 weeks?