It's a conditioned response. Low pressure used to trigger my migraines and fibromyalgia. I researched it and found that there is enough change in the body from the pressure change for you to notice it, but it is no reason for pain. Our brain has linked the two like Pavlov's dog linked the ringing of the bell and food, thereby salivating at the sound of the bell. I was conditioned to link the subtle changes in my body from barometric pressure changes to pain. I overcame this by talking to my brain when I sensed the pressure changes. I told myself "there is no logical reason for this barometric pressure change to cause pain. So just knock it off." It eventually started to work, but I needed to catch it early. I'm not bothered by it anymore most of the time. Occasionally I'll notice a twinge, so I just talk to my brain. I guess this works because eventually logic and reason from the conscious brain overrules the unconscious.
Hope this helps.