I think i may have pulled a muscle under my shoulder, when i stretch my arm it hurts, i lift weights would it be best if i stop till it feels better?

Sprain/rotator cuff: The most common cause of shoulder pain and impingement of abduction and overhead reach in the setting of weight lifting is a rotator cuff tendinitis or tear of the conjoint tendon. A PT can evaluate your range of motion. Rest, ice and topical NSAID creams and patches available at drugstores are very effective. If impingement persists you may need an MRI and sports/Ortho referral.