The roter cuff in my left shoulder started hurting earlier today, and I didn't do anything that might have injured it. I tried to lift weights (work my chest), but that only made it worse. It's hard to even lift my hand to comb my hair or brush my teeth. Any idea about what can cause it to hurt for no reason at all?

What I'll do is for the rest of the day, I'll pretty much just keep my left hand at my side and try not to lift it for any reason, and see if that helps.