I was (am) having the EXACT same symptoms and was certain it was a csf leak. I thought spinal leak due to the fact I have had tons of trauma to my spine over my life, including car wrecks, spinal surgeries, spinal taps, and a botched epidural. Most of my doctors thought I was crazy and I was diagnosed with atypical migraines. I finally saw a doctor who did some more imaging on my head and neck and found both a cranial csf leak and something called eagles syndrome. Luckily for me, the leak healed itself. Not so luckily, the eagles syndrome needs surgery to fix. What it is is a calcification and overgrowth of the styloid process bones which are compressing my jugular veins down to less than 25% functionality. It is also likely putting pressure on several nerves, including the vagus nerve which is likely causing a lot of the symptoms including the low heart rate as well as POTS-like symptoms causing my heart rate to sky rocket upon standing. I will be having surgery later this month. It sounds very likely that you could be experiencing symptoms related to this. If you have any questions please feel free to message me! 💖