I have had this for the last 10 years. What they thought was migraines turned out to be cervicogenic headaches - although my head didn’t ache! When I have hormone volatility it gets much worse and, interestingly, when I force stretched it a couple of years ago I overstretched a tendon or ligament and had to wait for 3 months for it to heal and all my nausea and dizziness came back during that time.
My osteopath said I have a scoliosis (my collarbone has always looked wonky) which then puts pressure on the nerves. I now lift weights to try to keep the area better supported. Apart from that I have to live with it.
So I think, for me, it’s blood flow issues from cervical compression, which makes me ultra sensitive to hormone fluctuations. This also affects vagus nerve and digestion as everything is connected.
Perhaps all of us who cannot tolerate hormonal fluctuations, including HRT, have a structural cause that becomes more apparent as we age?