I have had migraines for many years now and recently found a knot on the back of my head. Could that be what is causing migraines?

Occipital neuralgia? If the "knot" is a tender point right at the back of the base of the skull, you might have a condition known as occipital neuralgia. Patients with this condition will feel pain shooting up into the scalp when pressing on the occipital nerve, and can often get marked relief with an injection of numbing medication.
Potentially. See your local headache specialist, who will be familiar with occipital neuralgia, to see if that is the root cause of your headaches. If so, the "knot" may be something contributing to your headaches.