Did they do an MRI of your arm or of your spine? I'm not a doctor, there are seperate doctor forums, but if I had to hazard a guess, I'd say you have a pinched or trapped nerve, maybe a bulging disc.
It's surprising the number of people who have these to a mild extent. I would advise seeing your doctor and maybe asking for a referral to a neurologist. Neurogists deal with nerves and it sounds like you have a nerve issue going on for sure.
I would also agree that you need to see a doctor to get a x-ray or mri, cat scan something to look at your neck and your back. I've got degenerative disc disease (quite a few people on this forum do) and one of my symptoms is arm, shoulder pain, pinching, feeling like a blood pressure cuff is squeezing on my arm. I don't know if those are some of your symptoms but if your having any trouble with your neck, too I'd definitely recommend taht you go see someone.
As you probably know, we are not doctors here, but most of us are chronic pain sufferers and there is a lot of knowledge on this board. So please post away, ask questions, we will be happy to answer what we can and assist you in any way. Welcome, and keep posting!