Is it bad to lose.. 6 pounds in 6 days?

I’m on a diet and I’ve been eating healthy and exercising a lot but apparently losing so much weight is unhealthy. But I don’t know what to do differently. I’m not starving myself at all or doing crazy workouts. Also, my eating habits before were horrible and I’ve never exercised! Like.. late night fast food, eating a big meal a day.. stuff like that, so maybe my body isn’t used to the change.. help?