Mine was serve tendon inflimation at the wrist. and actually thought I had a cist in there.. I had a hard knot form and it was very painful. Then the pain worked its way into the elbow. Thats when I stopped and took a year off and started playing and teaching left handed. I dont use poly anymore.
I should say that my wrists and arms are very strong so that was not the issue. My rackets are 12.5-13 ounces and very flexible. My technique is pretty much perfect and I dont arm the ball but use rotation of the body to swing the arms.
Its a lot better now. Wrist is fine. Elbow is getting there. My left arm is 100% even though it was weaker and has stood up to rediculoussly long hitting sessions and other training. I just dont use poly anymore.
That dang string almost took me out of the game. I thought I was going to need surgery to fix my issues and I am not one that likes to be cut.....hence the swicthing to the left hand.
I would try takeing a little time off, maybe playing lefty if you can deal with the frustration (hey your still playing and it will help balance out your body so to speak), and stay away from Polyester strings reguardless what people tell you. They did not do anything for my game anyways. I just thought the durability was nice. Volleys are for crap with that string.