What could case the pain in the front of my shoulder and it only hurts when i drop my arm?

Shoulder strain: It sounds like you may have a pulled muscle or shoulder strain. If pain, can try over the counter Motrin three times daily with food for pain and inflammation for the next few days. Recommend gentle stretching/range of motion exercises to stretch the shoulder and keep it moving well. May apply ice for 15 minutes at a time, can alternate with heat. Symptoms should improve/resolve within 2 weeks