This may seem an odd question considering you are not overweight but have you been trying to lose any puppy fat? Its just that when i was 16 i decided my stomach was too big (it was'nt but still) so i stopped eating as much as regular and then developed a strange craving for fish-cakes.
I do know that if as NJQT said you are lacking your iron you can at least check to see if you are at an anaemic stage by looking at your eyes in the mirror, if the whites are looking yellowy then start taking some iron supplements, i know that not all multivitamins actually include iron so check yours do.