Yours sounds like it comes and goes. Mine comes and stays.

I just got my new exerciser that I ordered and really want to use it but right now my left arm is too sore. I use my left hand and arm a lot since my right hand and arm are numb. Wonder if it's just strained from overuse? It almost feels like a bruise (tender to the touch) but when I lift something with my left arm it hurts inside.

Just one of the many joys of MS.
I'm glad yours has calmed down. It sort of sounds like a pinched nerve. Pain is not fun.