You'd be very constipated for the first week. Then your body would start eating itself due to lack of nutrition. You'd be very weak. Basically at the end of the month you'd look like a druggie.
I used to be the same way about eating in front of boys from the age of about 13 to 17. I didn't want boys to think I was fat or gross by eating. So I'd wait til I was home to eat. You probably either do the same because you are self conscious, you don't want to look/seem fat.