Have to walk 20kms a day, eat 600 calories and weigh 41 kilos, now been prescribed quetiapine and fluoxetine , will this give me weight gain, scared?

No one should be: eating only 600 calories a day and exercising as much as you do. That is a starvation diet. Your fatigue, poor concentration & lack of pleasure may be related to starving yourself. Please consider working with a psychologist who specializes in eating disorders and a dietician to help you. The physicians here can tell you RE: medications