Mighty Matteo, thanks! I had a suspicion it was related to serves and more specifically kick serves. Other than rest, you should try stretching it before playing, it helped me anyway. But I guess the real solution is to stop arching the back so much on the serve... I wonder if other strokes are also involved.
Ramos77, I've seen physios, osteos, chiros, ... everything. IMO they all have their merits, as each has a different approach. The problem is that it is very rare to find someone that's knowledgeable about tennis. So they'll help you fix the pain, but they won't help with the underlying problem that's causing it (tennis).
LeeD, yes, I usually play on painted cement courts.