I've always been taught that a good treatment should not cause you any pain, it is only treatments that are done incorrectly that should cause you problems.
This doesn't mean that you are doing things wrong, just that you can learn different techniques that put less stress on your body when you are delivering a treatment - so, for example, with deep tissue massage, you make better use of your body weight than your strength, and adopt stances and positions that spread the strain over a far larger area, rather than just making all the effort with your wrists.
I guess it is like the H&S guidance on lifting heavy items - you can lift safely if you follow some simple rules, but it just takes a while to learn the rules and make sure that you follow them every time. At first, the different positions may feel awkward, but with a bit of practice, they can completely change the way you feel after a day of treatments - sure, you might be tired, but it should be mild ache rather than physical pain.