Hello, Thanks for your query. It could be biceps or rotator cuff muscle strain. Any strained muscle when stretched gives severe pain. I advice you to follow the below mentioned guidelines - * Give adequate rest to the involved muscle group and avoid any movement that causes pain * Ice packing once in every 2 hours over the affected area * Moist heat application to relax the muscles * Local application of muscle relaxant gel * Analgesic medications as needed I do hope that you have found something helpful and I will be glad to answer any further query. Take care