Yeah as you seem to have discovered, it's very important to make sure that your left arm is straight before you begin to draw (pull) the bow. If you don't do this it means that you will have to do two things to draw the bow, pull back with your right arm (using back/shoulder muscles) but also push forward with your left which in a short period of time will do some damage you really don't want to do. Take out one of these actions and it'll make life a lot lot easier.
So remember, left arm straight before pulling back. The second thing I want to point out is how you grip the bow, I'm not sure how you're doing this now but I'm going to point out the obvious just incase you're doing it wrong. If you've got it right first time then ignore this.
You want to make sure that the bow in your hand looks something like this or close to it.
Notice how the bow rests against the fleshy part of my palm/thumb just underneath the thumb. I've seen so many people twist their hand and have the bow resting against the thumb it makes me cringe. To rest it against your thumb is to strain your thumb and the muscles holding it there. If you rest the bow against the fleshy part on your hand then you can see in the picture how the force of the bow is directly along my arm, not at an angle to it. This might help get rid of that pain in your arm.
Hope this helps.
Gino.