Anyjoe is right and so is soucerer. If you don't like it then just stop going. When I did process work, I used to have a massage each fortnight for my back and shoulders but couldn't help getting hard. I felt very self-conscious but one day the masseur said, 'relax, its normal, do you want me to finish it off?' It was a great way to finish off a really good massage and apparently it happens a fair bit (Can any one confirm?) I think you should stop worrying.