My guess is that you are hungry because it's your first day. I read somewhere (don't have the reference handy, sorry) that your stomach does stretch and shrink over time; if you were previously in a state of feeling full and satisfied most of the time as a result of eating more food, regardless of what its content was, it is now having to get used to feeling partially empty. The hunger pangs subside over time, and that is thought to be a result of the stomach shrinking.
Dunno if science supports it, but it makes sense. It also seems to be pretty consistent with my own experience. Some days I eat a lot and feel really full; the next day you would think I would wake up and still have "leftover full," but I'm actually growling.
Congrats on the good day so far!
And now that you don't have to be perfect, you can be good.