Suggest Remedy For Soreness In The Armpit

I woke up with a sore right armpit in the high crevice closeer to the front of my body. The next day the pain spread into the inner part of my bicep. It feels like a rubber band is wound too tight between the two points (underarm and elbow/inner bicep) and it is a pretty harsh pain when I extend my arm all the way out. Any ideas on what this could be? I have no swelling or bumps/lumps but I do seem to have a tracking like bruise from armpit to middle of bicep.