Hello and thank you in advance for your help...

I am 51, in menopause, a former athlete and have gained twenty pounds over the past three years that I cannot lose. I am currently in Ketosis and was losing until I hit 151 pounds. Now I am bouncing up and down for the past four days. I have been eating LESS THAN 20 carbs, LESS THAN 1000 calories and burning an average of 1800 calories for ten days! I am starting to get headachy and lethargic but I am sticking to it. What is going on?! I should be dropping weight steadily and quickly. So discouraged. This is too hard for no results.