It could well be, that you are missing iron in your diet.
I'm not a vegetarian, still less am I a vegan (and nor will ever become one - I cannot conceive of a world where dairy - especially cheese - and eggs no longer play a role in my diet), actually, these days, I eat very little meat, and go for weeks (closer to months, at this stage) without meat.
Until I start craving, dreaming of, fantasising about, thinking of in idle moments - treats such as Iberico ham, or fillet steak, rare lamb cutlets, ham hock, poached chicken thighs, sausages, chorizo with my fish, or even - perish the thought - gourmet burgers; that is when I realise that something (possibly iron?) is missing on my diet, for such sustained and serious cravings usually feature when greed transforms into need.