I had the same problem...its just where your body is changing. If it feels like a pulled muscle that's where your lil miracle is pushing everything around and making room for itself in its temporary home:) You can talk to your ob about it if it makes you feel better. Never too many questions!
I wouldn't hesitate to call your doctor anytime during pregnancy, they expect it. Most likely it's pressure and position of growing baby, changing sleep position will help, you shouldn't be laying on your back, left side is best for blood flow. Give him a call, you'll feel better. Congrats!
Don't lay on your back. It's bad this far along. The baby's weight puts pressure on a certain vein and that it cuts off blood flow to your legs and to the baby. Laying on your left side puts less pressure on it and laying on the right puts none at all. Do not lay on your back. I already knew all this while I was pregnant and while in labour, the nurses told me that so it was confirmed what I already knew. right side is best