Some possibilities: adjustment of your metabolism to less calories, fluxes in hormone levels, depression, stress, side effect of medications and antibiotics, etc. Sometimes there’s no feasible explanation at all, but if you are under eating considerably, see a doctor to make sure you don’t have any underlying physical issues and for advice on how to get your appetite back.
A message from Anonymous
For the last couple of weeks I have not really had an appetite. I used to binge heaps and the taste of food would just be amazing. Now I just eat because I have to, not because I want to. I'm not sick and I still do my boxing and running but could you tell me why i've lost it?