Mid left back shoulder muscle pain while playing volleyball

I play a lot of volleyball and I am a right hander.  I am in shape, never smoked, never drank. 5'10 190 lbs.  I have very broad shoulders.  

The describe the muscle I am talking about put your right hand pinky of your left shoulder and reach your fingers twoards your spine, kind of at a 45 degree angle (not close the spine, not close to where your fingers would be if they were straight, but inbetween). I can feel it tense right now and I havent even been playing volleyball today.

I am trying to determine what I can do to prevent this shooting pain that happens after about an hour of playing volleyball.  If Iundge forward and the muscle is bothering me, it shoots pain like a cramped muscle, just in that area.

If driving home after vball and I look left in the car at a stop sign, it triggers the muscle cramp.  ON a pain scale of 0-10 its a 9.5 and can take me to my knees instantly.  I've talked to physical therapists and their answers are a bit to vague.  I type on the computer all day and I try to keep the best posture I can and it only bothers me when I use my right hand a lot during volleyball.  The only thing I can do to get rid of the pain is try to massage it while laying all the weight of by body on a tennis ball, on that muscle.

Unfortunately I can never afford a massage and I cant afford drugs and am looking to just improve my stretching (which I cant stretch this muscle for the life of me and i've seen 3 people that have tried to get a good stretch for me) or doing whatever it takes to prevent this pain without spending a crapton of money that I dont have.

Thank you.