When I am in shape I experience it on an ongoing basis, as did/do many other racers. When your resting heart rate and blood pressure are extremely low it is unavoidable. With a HR of 40 and a diastolic pressure of 70, standing up quickly means it's going to take a moment for your system to catch up. So you get used to getting out of bed, standing up, etc. slowly.
Beyond that, however, I am sure you will get a litany of responses to the effect of not seeking medical advice from a bunch of strangers on the internet, and I concur with it.
Dehydration can contribute to hypotension, perhaps that would explain why you feel better after a day or two, after you replace the loss of fluids. You should try and limit your pre to post ride water weight loss to perhaps 1%.