Why do I have so many knots in my shoulders and back?

I get a lot of muscle knots in my shoulders and back, especially when I have a migraine, but only when I do. I wonder if these knots are actually causing my migraines? A chiropractor that I saw a long time ago was the first to point out the knots, and my boyfriend tells me that my muscles are so knoted, they feel like bone! Im just so tense, and I cant understand why.

Is their any reason for this and is there some thing I could try to do? Ive tried a lot of things, but they only temporarily help, since the knots just keep coming back, probably every day.