I’m having a problem with my left arm.

3 days ago out of nowhere I started having pain in my left arm. It feels heavy and hot and like it’s asleep but my hand feels cold. If I hold my arm out straight I can lift it fine but if I toilet it sideways I can’t lift it at all bc the pain is too much. I can’t put pressure on it or lift it sideways and the pain is more in the upper side of my arm above where my elbow area is. It’s been 3 days now and no improvement ... what could be wrong?

I’m 21