I have a question. This pregnancy is completely different so I feel like I don't know anything 😂 so…

I have a question. This pregnancy is completely different so I feel like I don't know anything 😂 so I'm 17 weeks (almost 18) every once in a while in the exact middle of my lower stomach it gets super hard. So I push on it gently and it starts hurting real bad for a few minutes. Any idea what it could be? 😣😞