Eating disorders can occur rapidly and they can be very dangerous. People do not realise that making themselves sick, or becoming severely underweight could lead to serious heart complications. They do not realise that being extremely underweight or losing weight too rapidly could become fatal.
Please do see your GP urgently if you think your child is developing eating issues to get a referral to a psychiatrist. They might well need close physical health monitoring and it is certainly in the best interest of your child to get help quickly before the issues become too severe or entrenched. The young person is likely to tell you they are fine, there is not an issue and they will start eating normally again. You may wish to believe them and hope this is the case but often this is too hard for the young person to cope with alone and they might just become more covert about their behaviours. Do not wait until these issues have spiralled dangerously out of control.