What Causes Pain In Arm Radiating To Elbow?

I am having pain in my left arm. It feels like a spasm. It starts in the back of my hand and radiates to almost my elbow. The first time I thought I slept on my arm the wrong way. I was in the shower the other day and the pain came on suddenly and I had spasms for about 10 mins. I also had one to night. They are getting more frequent.