Title says it all - when I do trills I get pains in my left arm, in particular, down the side of the arm. What's causing this and how do I get rid of it?
Curses...i have nothing of interest to write in this here place.
If this is occupying just when you practice, take breaks. Not go play video games. Takes breaks means you still are practicing mentally, but not physically.

If this is occurring every time you play a trill, how is your technique? Do you have too much tension in your hand or on your strings? Can you fix it? Etcetera.

I'm sure someone who knows how to do trills technically correct can help you further, but this is a step in the right direction at least.

Ultimately, the one thing I want to press, is to stop playing if you feel any pain.
Hope I helped a little,