Diet then crashed (question)

I recently started a 800 calorie diet and have been losing weight quickly but also crashed… second or third biggest crash since my initial one back in 2019.

I was on the diet (all foods are open to eat but limiting calories) for three days when the crash came on which makes me wonder if my weight gain had something to due with nutrition deficiency to begin with. The only change was my Metrx protein bars for lunch however they are loaded with vitamins and minerals. Has anyone had any issues with metrx protein bars?