I think I'm starting to relapse after 20 months (being pregnant and the last 11 months of breastfeeding)

I made sure to stay healthy for my son but I'm feeling so down about myself and I can't eat above 900 calories. And I mean I latterly can't. I thought I have been. I started tracking again and the past week it's been 800-900 calories a day. I've been trying I push myself to eat more and i just feel sick to my stomach and I can't spend all day feeling sick when I have my son to take care of... I don't know know I'm going to do...