I'm eating badly but losing weight slowly?

So a few weeks ago i started going for walks everyday and eating better and started to eat better and only drink water and my weights stayed pretty much the same... And these last few days i've gotten a bit lazy and havent been for walks and ive even drank coke and had macdonalds and kfc twice and my weight is slowly going down... I like whats hapening but i dont get it? shouldnt i be putting on weight? (oh and i have never smoked or taken drugs in my live nor plan to).