hello,
I can see one or two possible contributory factors. Firstly low-lying cerebellar tonsil are crowding the opening at the bottom of your skull (Foramen Magnum) which may be interfering with your cerebro-spinal fluid flow (CSF). You also have a retroflexed odontoid (an anatomical difference in the peg-like bone near the top of your neck, where it points slightly backwards instead of pointing straight up). This in itself doesn’t necessarily give rise to problems but, if you have been involved in a trauma and damaged your head or neck, or have the joint hypermobility type of Ehlers Danlos Syndrome (EDS), then this could be contributing to your problems.
Until you find out for sure I’d be avoiding any slouched, slumped, or leaning forwards positions, try and sit as upright as you can, with your head balanced above your shoulders. Use a pillow that supports the curve of your neck and don’t have any pillows or cushions pushing on the back of your head.
Regards
Barbara.