Some people with migraines have very tight muscles in their neck.  Stress and bad posture can cause the neck muscles to tighten up.  This can put pressure on the area where the muscle attaches at the base of the skull.  Once the muscles is too tight, it can put pressure on the nerves leading to the brain.  This affects nerve and blood flow.  This can easily cause headaches.

Shoulder tightness also put pressure on the nerves.  The shoulder muscles attach to the spine.  These muscles often affect each other.  Once you get a muscle spasm in your back, it can affect muscles all along the spine, leading to the neck and the base of the skull.

I am not giving you a diagnosis for your particular problem.  These are just observations I have made.

I had a patient once who was being treated for migraines between her eyes.  She had been going to a pain management clinic for years.  She was very healthy overall, which is rare.  It was a simple case of a blocked meridian for her.  The acupuncture meridians are how energy/blood circulate in your body.  If you have an injury somewhere, that blocks the blood flow.  In her case, within a few weeks she was completely well and got on with her life.  Most people take about 2 months to address all their health issues though.