Upper left back pain?!

hey i play tennis quite a lot(4-5 hours a week at the least) and after most of the 1 hour, 1!.5 hour tennis session, I always seem to have pain, stiffness, in my upper left back(near the left pointy bone)!.!.!. and feel pain when i try to steer my head to left!.!.!. anyone knows why it always appears there!? and how I can avoid it!?

I use:
luxilon big banger strings, very light weight wilson k zen racquet(is this why!?!?)
I play with:
- 1 handed back handed

thnx a lot!Www@Answer-Health@Com

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