Storytime Shoulder

Now, I don't know if anyone else gets this, because I've never heard anyone else discuss this before. But, I've been having trouble with the muscles around my shoulder blade. Specifically on the right side. I have come to call this "storytime shoulder". I'm sure it's partially due to my poor posture, and my tendency to get so excited that I lean closer and closer to my students while trying to engage them. I've been trying to sit better and switch holding books on my right to holding them on my left (but this is a bit like trying to use a mouse with your non-dominant hand). I'm sure conducting storytimes for 3 classes in a row (1 1/2 hours straight), does not help. Any tips or tricks? Perhaps I'll get some insight into my posture in tomorrow's paper. The local photographer popped into my storytime this morning and disruptively took pictures for five minutes. Very distracting for me and the kids. I would've liked it if she had at least asked permission first.