What Causes Stiffness In The Muscles On The Back?

hello I am a 39 year old woman. Over the course of the last 10 years my muscles in my back and shoulders have been becoming tighter and tighter. In the last two years it has been spreading to my hamstrings and calf’s as well as my arms. In the last 4-6 months the muscles in my back have become completely rigid with spasms occurring more frequently. There have been a couple of times recently that the spasms have been so hard they tighten around my ribs to the point that it is hard to expand to draw in a breath. My doctor just had me tested for stiff person syndrome but we have not received the results back. Are there any other possibilities that it might be? Thank you.