Pain from Tennis

I have had severe pain when serving in Tennis for a long time now.
The pain is located underneath my shoulder blade.I have tried serving more gently but it seems that the pain can still return.
Its almost as if suddenly something changes and the pain is back. I took a 7 month break from Tennis.
Tonight I made a decision to give up tennis as after seven months break and then 2 months playing again the pain has returned as bad as ever I don’t really want to give up as I play at top club level and do enjoy it, but I dont want to end up ruining my shoulder for when I am older. I am 34 so injuries don’t heal quite as quick as they used to.
Its very odd because the pain can go again and at times when I have served even harder in pure frustration it can not hurt at all.
It seems linked with rotation but I have had little or no help at all.
Last time I went to the doctor he asked me to lift my shirt and injected Cortisone.
Being in the UK physio is given to people who are alot more injured than me and I get sent away with pills etc.
If it helps at all I am left with a numb bruised sensation in the same area after playing.
I am not going to play again for the foreseeable future but I would like the opportunity to maybe excercise to see if I can play again some time
Thanks in advance
Chris