This is far far too low. Caloric restriction should NOT be paired with fasting.
You should be eating this much normally after breaking a fast. By artifically restricting your caloric intake to 1000 you're driving your Base Metabolic Rate down towards that 1000 calories.
If you give your body a 'budget' of 2500 to 3000 calories your base metabolic rate will adjust to that new budget.
The reason you put on weight easily is you've taught your body to store anything in excess of 1000 calories as fat.
When you fast, fast. When you feast, feast!!
I broke a 60 day fast today. I'm shooting for 3500 to 4000 calories a day for the next week. You need to stop artificially restricting your calories. It'll take a couple of fast/feast cycles, but your body will adjust. You might gain some weight, but that's going to be the price of fixing your metabolism.