R.D.
Where is the pain? Is it on one side or both?
Aside from a muscle issue, it could be an ovarian cyst, kidney issues, or even something with your legs or feet (or holding a baby on one hip only)... If you can function, I'd give it a few more days, then see a doc.
If your pain goes through the roof, obviously, get your butt to the doc sooner rather than later.
Ease up on the heat, since it can inflame already traumatized muscles.
Feel better!!