Sounds like Brachial Plexus/Radial nerve impingement. Do some dislocates with a broomstick really emphasizing the pec stretch. See if that helps. Also! Make sure you are sitting tall when rowing. Don't let the posture slump furward as this can exacerbate impingement.
"Survival will be neither to the strongest of the species, nor to the most intelligent, but to those most adaptable to change."