LozF,
I wish I had seen your post earlier!
I had a similar problem a few weeks ago. It was mainly a numb or tingling left arm and a sore shoulder, but because the pain got so bad, it also started to restrict my movement. It was possible to move my arm anywhere that I wanted to, but it was damn painful and it felt most comfortable to hold my arm as if it was in a sling.
I went straight to a physio who did her thing and reduced the pain immediatly. I had three treatments in total over about a week and after the last one I was completely cured.
My problem was that I had tight shoulder and neck muscles, and this caused a nerve in my neck to get pinched - hence the tingling fingers. I have no idea what I did to cause this, I just woke up with it one day.
Physio suggested that it could be exercise induced as I had just started cycling and I tend to grip the handlebars very tightly. Alternatively she said that it could just be stress from work and life in general. I tend to store my stress in my shoulders so this would seem possible.
If the pain doesn't go away soon then I strongly suggest a physio. GP's are great with infections, but unless your GP has an interest in sports problems, they don't tend to be so hot on helping with muscle issues.
In the meantime take ibruprofen (unless you know that you ahve a problem with it) to reduce any swelling and relieve the pain.