I had something under Arimidex in my hands which went away on Tamoxifen - I've mentioned it before, it was trigger fingers/thumbs, which actually means the tendons have swollen. I am now on exemestane (Aromasin) and have had just the odd twinge. I was told it could be operated on - they open up the sheathe around the tendon (I hope tendon is the right word - my anatomy is not very sound) and it grows back together larger. But I have also met someone who had it and it went away.
It was a pain that started first in the right thumb - I'm right-handed and whenever I grabbed something it hurt. Maybe this is what you have, or maybe not - I realize bone pain is a more common side effect, but trigger finger is a known side effect of AIs but not Tamoxifen.