I get those spasms too. I just got one a few days ago right under my boob! It was a muscle spasm because I was trying gentle yoga streches for the first time in a long time and I over did it. So for me, it is part FM and part overstressing the muscle. I see that you are on Baclofen, when this happens I take an extra dose with some ibuprofen to help with the pain and spasm. Just make sure you aren't going anywhere for a while when you do this. I did this just on Tuesday and that muscle still hurts like the dickens!