I also started to suffer from dizziness and lightheadedness since I started being peri and it was bad enough that I fainted twice. It was always worse first thing in the morning. Then about 8 months after I got a fitbit, I finally put two and two together. My resting heart rate is naturally very low and when it gets below 51, I started getting some of the symptoms, but what I noticed is that about a week before my period, or when I was taking hrt, a few days after starting the progesterone, my resting heart rate would gradually go up, until a couple of days after my period started when it would suddenly go back down very low and that's when I got the worse of it. I looked back over the past year and sure enough, it's been the same each time.
I've concluded that my heart rate is linked to progesterone and when it drops suddenly, my heart rate is too low, hence the symptoms of dizziness, lightheadeness, buzzing in my ears, and an exhaustion that just totally floors me.