Well considering you are a recovering anorexic although 1000cals is not a lot of food for the average person, it is for your body because it’s so use to very few calories. This energy change could be one cause of the weight gain.

This is very untrue and harmful to say. Yes, someone with anorexia will have a reduced bmr and overall tdee in response to starvation into the underweight levels, but there is no adult on the planet who is gaining weight at 1000 calories a day. You should not be telling someone with an ED that eating 1000 calories will cause them to gain weight. It's not possible, and it's only going to risk driving them into further restriction.