Hey J.,
About a year after my c-section I had the same feeling... but it was a cyst on my ovary. My guess is that they would have seen that if you went in... they can be very painful. I went to my GYNO to get checked out because they are very familiar with the lower part of you, plus it is cheaper to go to them then the ER. If you have not talked to your OBGyn give them a call and ask their opinion, they may say come in and give you an ultrasound to see if they can find anything.
Otherwise if you have done that and it is just a muscle strain that is week find some simple exercises to do for that area. I would look for ones that don't put strain on the muscle, you may have to take it slow to get it back to full strength.
Good Luck... hope you figure it out!