My mob farm is all lighted up but there are no lights in it

So i made a mob farm that used to be all dark but now its all lighted up even tho there are no light sources in it, if i press f3 it says sky light 15 block light 0 what is really weird i think because it is all sealed off of course. the farm is made out of netherrack but i dont think thats a problem. i am also not using some kind of mod so that everything is brighter.