Shoulder pain when playing theremin?

Hi! I recently bought a theremin (moog theremini) about a week or so ago, and I've been loving it! However, I've noticed that my shoulders have started to ache a bit after I play--I can't tell if it's because I haven't quite gotten the hang of the technique right, or if's perhaps because I'm sitting and playing? I don't want to get into a habit of having a bad playing posture so I don't end up accidentally injuring myself. Has this happened to anyone else or have any tips? It would be much appreciated. :]