@josieglow Please don’t take this as medical advice, but what you describe is similar to what I was experiencing. I now use azelastine, an antihistamine nasal spray, that works for vasomotor rhinitis, non-allergic rhinitis, and sinus pressure. Antihistamines are known to narrow blood vessels. I believe that is the drug action that stops my headaches.

I am on tamoxifen which is a vasodilator (vasodilation is known to lead to hypotension). I had headaches similar to yours, first documented in March which seemed insignificant at first. I eventually had a brain MRI as they searched for reasons for my headaches. It was decided to lower the dose of tamoxifen, that helped.
Yet, when March came around, the headaches started again. I did not have noticeable sinus congestion but I had been through an viral upper respiratory infection recently. My doctor prescribed azelastine 1%, but it can be purchased OTC as a 1.5% solution (long term I’d rather be on the 1%, and health insurance makes it cheaper than buying OTC).

You had mentioned the changes in barometric pressure, which for me was possibly the repeated onset in March, but of course there are a lot of allergens in the air at this time, too. I also had a feeling of fullness right near my left ear, intense headache upon waking, headache through the day, and my tinnitus increased a lot with the higher tamoxifen (vasodilation?) dose. So along with barometric pressure issues, your possible inter-cranial hypotension sounds similar to my symptoms.
Anyway, this is what works for me. Hope you find an answer!