hi - it could be many things. Honestly it is hard to say what it is...
1. Muscle strain/fatigue
2. Nerve strain
3. Shoulder impingement
4. Wikipedia reference-linkRotator Cuff dysfunction causing any of the above
5. rib dysfunction causing any of the above
6. referred pain from the neck
7. AC joint issue
8. tight shoulder capsule
9. Fatigue causing a strain causing any of the above
10. Poor technique causing strain

the list goes on and on.

Seriously, get a good assessment and diagnosis if it continues to bother you...