Help me please,

I have a question to anyone else who this has happen to them. This was the first night I was on top while have sex with my fiancé and being almost 6 months pregnant (on Wednesday I’ll be 6months pregnant). I’m going to go into a little detail:

We were on the couch and he was laying/sitting down and while I was on top I felt pain all on my lower abdomen going all across from left to right. It felt as if I did a lot of sit ups the night before and i was sore but that wasn’t the case. So I told him and we stopped with that.

Has this happen to anyone else before? Or is there a reason why this happened to me. I got scared but then it went away already it doesn’t hurt but I want to know if this is common to other ladies out there.