What does it mean if you have a period for a day? especially if you have secondary amennorhea caused by weight gain.

It may not mean: anything more, especially in someone who has amennorhea. Your hormones apparently do not rise and fall in the normal pattern leading to no menses; this same phenomenon could cause only 1 day of withdrawal bleeding. Because it is atypical, if you are sexually active a pregnancy test should be checked. Just continue under your doctor's care for the amennorhea problem. Good wishes:)