I dunno. I'm not a doc. Lol. It could be a number of things. Like sleeping funny or sitting to long, or not using the potty frequently. I would talk to your doc if it becomes a hinderance to you though.
I was pretty thin (like 110) before pregnant, and I've gained over 20lbs so far... maybe it's because I'm gaining so fast?
I am but I'm father along at 32 weeks. So I dunno why you would be havin it so early unless you have small bones or your baby is snuggled to close. Ask your doc about it, it could be a pulled muscle too. Wouldn't be to worried about it till you get an idea from your doc. Take it easy though so you don't strain it to much.