What Causes Pain And Indentation In The Upper Arm Post Weight Lifting?

I have been weightlifting recently and everything was going fine until I felt the strength go out of my left arm. I went home and realized an indentation in my upper arm and a dull pain around the same area. It does cause a small amount of pain whenever I use that arm. It also feels like the muscle has moved where the indentation is in my uppet arm. I also cant seem to flex my biceps in that atm due to lack of strength. Any ideas on what I might need to do or how to get my arm back to normal. thank you