I never seem to put on weight anymore?

Since I've been at uni my mum says I've lost a lot of weight, but thing is I still eat loads. My university is quite hilly and you have to walk about a lot, but surely it wouldnt have affected my wieght that much. I stay pretty much the same weight now despite eating crap. I'm 19. Should I be worried? I want to eat healthily and am not taking advantage of the fact I'm not gaining weight.