Chances are there is something in your work that is challenging your body in a way your body is not stoked on. The key to longevity as a therapist: adapting to what your body is telling you and changing your technique and body mechanics until you can work without causing problems to yourself. Everyone runs into this, what it comes down to IS NOT being "this is the way I work a someones back, and I can't do it another way". Yes you can. There is NO good technique that causes you a problem, no matter how good it may be for your client.

I would guess and say your compressing your joints when your working, and probably using your strength rather then your weight as your tool. You can learn to work through what is called "open joints", getting the power of your integrated body behind your hands. I teach deep tissue and other advanced bodywork, and let me tell you, its all about your weight being your source of power. Size is not as important really, little people who use their weight can be quite powerful. I train therapist who have been at it for years and didn't realize how much strength they were using. Also the only way to work really deeply and keep a client relaxed is via being relaxed and using your weight. It really does feel different and is more effective.

I would also recommend finding a Rolfer to work on you. Working on your structure is going to help you out A LOT!!!!! It really helped me out, probably need to have your thorax decompressed and opened and help finding extension from your feet into your hands, in addition Thai should be able to help you open your body as your doing it. FIND A ROLFER!!!! Go to Rolf.org and see who is listed in your area, and ask them if they have experience working with arm problems and overuse injury. It might not feel great, but it'll really help you out, I promise. Rolfing is the shit, period when it comes to working out your kinks, and a whole lot more.