Back.. Muscle?

Hey ladies. Workout mommas. So I posted on here a few days ago about working out my abs while pregnant. They seem to be safe enough, not straining too much or getting overly tired. But now since I started every morning I wake up and the right side of my mid back hurts. What do y'all think that could be? Like I'm straining too much and need to slow down? Or like my muscles being sore and it's a good thing? If so .. Why only one side? Please help :)