I Have A Swelling On My Back, Right Side, Between

I have a swelling on my back, right side, between my shoulder blade and spine that started out small a couple of weeks ago but is approximately 3"in diameter now. The swelling has not changed the color of the skin, there is no redness however, it is getting more and more painful each day. It started out as an occasional ache but is there several times throughout the day now. Sometimes all day long. Sometimes it feels like a burning sensation, sometimes it just aches, like a toothache, and it has also felt like an electrical shock. It feels like the ache is deep under the skin. What could it possibly be. My pcp thought maybe Costochondritis but has done an xray and has ruled that out