While lifting weights, my collar bone/shoulder area suddenly started hurting really bad. what could have happened?

AC joint osteolysis: Weightlifters more than most people can develop a condition referred to as weightlifters osteolysis-- a breakdown (arthritis) of the acromioclavicular joint (ACJ). Rest, ice, NSAIDS, injections (local steroid) can all help once diagnosis is made by X-ray and/or MRI. See an orthopaedic surgeon if pain persists for more than a couple weeks.