I had shoulder surgery and now i have blisters all over my arm and underneath it. what is the cause and how do i fix it?

Depends: Blisters immed after surgery usually secondary to excessive tension on superficial layers of skin. This may be caused by swelling alone or surgical tape placed in excessive tension to secure bandage. If this seemed to develop later could be superficial skin infection or cellulitis especially if area seems to be expanding. If this is the case should be evaluated by your surgeon.