I do a modify Prolon OMAD. When I looked at the Marco’s of the meals provided in the Prolon pack it was way too many carbs. I had the same symptoms before and figure it is going into and out ketosis every day. I’ve since modify to one meal a day, 800 cals, but with ketogenic macros (75% fat, 20% protein, 5% carbs). I also shoot for a glycemic load less than 10. What this boils down too is a Grilled chicken salad with an olive oil/red wine vinegarette and 6 oz of homemade bone broth. With this plan, I have been able to maintain OMAD and stay in Ketosis for prolonged periods of time. I am now down 3% body fat and 10 lbs in 4 weeks.
Also, make sure you are getting electrolytes. I am a big fan of Squincher Zero. About half a single serve with 16 oz water twice a day does the trick for me.
Good luck and keep trying. keep experimenting. It took me 3 months of fiddling with fasting before I found a plan I could stick to and get results.