Pain in armpit after Plan B

I took a Plan B pill last week, I'm currently 8DPO and my period is due the 18th. I'm experiencing a sharper pain in my right armpit. My breasts DO NOT hurt and I do not have any other symptoms. It's just the right armpit. I read the entire pamphlet and didn't see anything on this. Has anyone experienced this after taking a emergency pill? Could it be the hormones from the pill and/or my period coming? Thanks in advance