Maybe you are too restrictive and your body needs time to adjust, or the food choices could be better. Insulin might need to adjust (can spike during big feeding). If you haven’t started with maybe 6-8 hours/day for a feeding window, start there. It sounds like a shock to your body and your body is figuring out how to adjust everything else. Then, take away more hours until you are at a perfect equilibrium and be patient, then do it for life in terms of maintenance.
What’s your net sodium intake in your average OMAD? If you are sensitive to sodium, eating your entire RDA in one go is likely to cause a fair bit of water retention, which can spike blood pressure, I’d imagine.
Yes. I can’t fast that long or my blood pressure goes very high. I can do it for maybe a few days, but when I did omad for a couple months, even medication wouldn’t control my high bp. Now I eat in a 7 hour window usually and my BP is good, even without medication. For some people, fasting stresses the body a lot.