My arm started hurting after playing on the piano a couple days ago.

I've been doing everything that my teacher tells me to do when I play.

What part of your arm hurts -- the forearm or from the elbow to the shoulder? Both arms?

Is the piano playing a new thing for you?

Not really. I've been playing for 5 years now.

And it's my right forearm that hurts.

It only hurts sometimes, though. I just don't know why.

The right hand does much of the work with the melody in treble clef, so maybe the pieces you are playing now are more complicated, with more fingers involved in chords, stressing your forearm muscles?

Are you tensing up? I always tensed during lessons and ended up with muscle spasms.

Yeah,I'm working on a pretty hard concerto at the moment.