I've been having recurring pain in my lower armpit for a week. It got really bad the first day where I couldn't move. I went to the walk-in the next day where the doctor said it was a sore muscle. It has mostly gone away since but today it came back slightly higher up in my armpit. Not as painful as the first night but the most painful it's been since then. I just dont think its a sore muscle when I've been to the gym once all week (doing cardio) and been on two 3k runs.