Hmmm wonder if it's some kind of nerve pain. Do you draw with a tablet? I found that, after many years of using a tablet without issue, I started to one day develop pain in my elbow when resting it on the table. Like tennis elbow, but for illustrators I bought one of those compression sleeves (something like this, though not this exact one http://a.co/8NQa2yd) and it doesn't hurt anymore!
This is my dominant/right arm, though, not the left. Don't know what could be causing your issue but maybe this would help!
Also, just make sure your computer set-up is good...Good support in your chair, your monitors are at eye level, your feet are elevated if they need to be, etc. It could also be how you're sleeping. It's funny but your mattress/pillow can have a huge impact on your spine and these random aches and pains. It's all connected. SCIENCE!