Abs feel like I've been doing sit-ups -trust me, I haven't been!

Erica

I'm 6w+3 with baby number 1 and for the last week or so my abs, lower and upper, are sore when I move certain ways or lay on my stomach as if I've been doing sit-ups! When I look it up all I can find is info for later in pregnancy and stretching. I'm not worried about it just curious if anyone else experiences this and why it's happening??