The Short & Long Term Effects of Undereating Without Medical Supervision

In the short term undereating can have the following effect (this is by no means an exhaustive list):
Mood Swings
Fatigue
Constipation
Hair Loss
Menstrual Irregularities
Dizziness
Brittle Nails
Poor Skin Condition
Headaches
 
In the long term it can have far more negative effects, such as:
muscle loss
gallstones
electrolyte imbalances
organ damage
bone density loss
vitamin/mineral deficiencies
and whilst you might feel fine now, there is not always an advance warning of the long term effects.

*Losing Weight | Healthy Weight, Nutrition, and Physical Activity | CDC *Healthy weight - NHS