Why Am I Experiencing Pain In The Left Armpit?

I think i have a pinched nerve in my armpit on the left side. if i lift it too high i experience excruciating pain and if i touch a certain part of my arm pit it really hurts the dr has given me Flexeril and it's working for the most part but i don't know why i'm experiencing this all i did was sleep on it wrong 2 weeks ago and it's been hurting ever since