Woke up with pain inside my armpit, can't move my arm up or sideways without pain, read about cancerous reasons or heart attacks. help? 16 y/o girl

Not those: You do not have cancer and you are not having a heart attack. Common things are common, rare things are rare. Most likely you pulled a muscle in your sleep or you have an infection setting up in the armpit such as an abscess or a folliculitis developing. Pain is obviously not normal, so if it doesn't go away by the time the doctor's office opens, then call for an appointment to find out why...