Exercises Limited

When I hit 300 lbs., I put myself on a diet and exercise program that landed me in the hospital with half a dozen heart attack symptoms. I had lost 15 lbs. in two weeks and my heart reacted. The doctors were not happy! Now I am eating a few more calories and wondering how myfitnesspal determined that I had to stay 530 calories under the number that shows up in my daily tracking. Does that change as my weight changes? Due to four back surgeries and the resultant neuropathy, my walking is limited although I am trying to ride my bicycle five miles every day. I am not diabetic. I will be 70 next April.