Why did my doctor tell me not to let my weight change?

I went in today, as instructed, because I only bled for 2 days. He said that my cervix were still open, but everything else was good, all things considered. When he confirmed I had a misscariage, my mind raced and went blank all at the same time. I have so many questions now, and most I can find online, but this one. Why should I not gain or drop any weight? I am overweight, so it seems like only gaining would be bad.