I calorie restrict as well. I spoke to my doctor about a lot of things this time around. Including my knees and shins for running, back for working out, and nutrition. Calorie counting is fine and dandy, but if you do consistently find yourself under 1000 calories, you need to find a good snack to increase it. I log my food, and also watch my general nutrition. Like, if I have nothing but carbs, the next day I try to up the protein. If I have low on fiber for a while, I'll eat something that is fiber rich. Just try to keep a decent weekly balance.
My doctor told me to try snacking on occasion if I was constantly under. Being under one day, you're just not hungry, it's good to listen to your body. But, if you are always under, try some healthy snacks.
Apples and Peanut Butter
A few Almonds (Calorie Rich and you barely need any to get those extra calories in!)
Some whole wheat crackers (a few)
I also had a eating disorder, so I personally push to the 1200 mark, eating under at 600 or purging all the time has made me feel more proud when I'm eating a decent amount. I check in with my doctor and counselor as well, which I do recommend.
My doctor did say that one day going low will not hurt you, it's when we consistently deprive our-self. She also let me know that my body would start to demand food, just not the normal "growly" way you may expect. Like Fatigue, weakness in muscles, "shaky" hand syndrome (she called it something else....), and mood swings from not getting enough food. I was told if I felt these symptoms to get a good snack like a boiled egg or some fish to help.