Mine did the same thing and it hurt to take a deep breath.I found out my back was out had to go get it cracked.hope this helps.
I Have A Bruised Feeling In My Back, What To Do?
I have a bruised feeling in my back Left side around bra area
May be a pulled muscle. I take it there is no bruised appearance? If you are active, you need to rest it and try not to use it but it is your back so that is not very easy to do.