I think I'm right in saying that you can calculate it by the collision angle minus your facing angle. I think that works. If the collision hits you at 90 degrees (from below) and you're facing 180 degrees (left), then the hit came at 90 - 180 = -90 (converts to 270), which is your left, which is correct.
If the collision came from above (270 degrees) and you were facing left (180), then the angle would be 270 - 180 = 90, which is your right.
That works for my case at least, hope it helps :)