Hi, yep just the muscles, im finding i have the same thing this time around a lot earlier on i was kinda concerned about twins as im popping really early but U/S confirms its just one. I found when was pg with my DS that getting into a pool was an amasing feeling it holds all the weight for you and eases the muscle pain. if you have the availability to it i really reccomend it. a bath dosnt do the same thing coz you arnt floating enough. Also another thing you can try that was reccomended to me by a doctor is a massage pillow just one of the ones that vibrate they really work because they numb your skin and confuse the muscles into thinking there is no pain and the best part is they are completely safe during pregnancy. Good luck i hope you find some releif for the pain.
Have a great day
Mel