Sheps. If the pain is down the outside of the arm down the brachialis (between the bicep and tricep) you've manage to impinge your brachial nerve. The most common way of doing that is by injuring your rotator cuff - the muscle which moves your shoulder blade round and stabilises the arm in the shoulder. Ice the area where the arm joins on to the back, rest it for a while then get yourself a dynoband and do the tubing exercises in this link.

http://www.med.umich.edu/1libr/sma/sma_rotcuff_rex.htm

As Scrap says you may have have to see someone to check your spine and neck alignment aswell.