Radiation to the area of the axilla can cause damage to the nerves which may be delayed - I'n not sure how long there is between the radiation and the appearance of your symptoms. If a nerve was damaged at surgery the symptoms from that should have been present straight away.
An MRI may not be sensitive enough to pick the damage up, however an EMG exam will, and you should ask your neurologist about scheduling one. An EMG should be performed at least 3 weeks after the onset of the symptoms, for it to be of most use.
Hope it was helpful.
Auntteta