I Was Just In A Store And I Was Holding

I was just in a store and I was holding something in my left hand and when I bent it my arm started to shake. I put the package down and it still happened. Once i got back to work I let my arm rest on the desk and tried to bend it and it didn't shake as much but it did a little. I hurts right where my vain is in the middle. whats wrong? am I just freaking out? I don't have ins so I can't go to the ER.