I've dealt with this issue on and off for quite a while. Have you thought about starting a mood journal along with your food/ exercise journal? It doesn't have to be a lot of work, but maybe it would help you see where it's coming from. It can be really short, just track the amount of hours you slept, your mood at intervals throughout the day, whether you're PMSing or not, what kind of vitamins/ medicine you took, and any other thing that you might be affecting your mood. After a couple of weeks or so compare it to your food and exercise journal and see if you can find any connections.
I've heard that weight loss and exercise have an effect on your hormones, so it could just be our bodies adjusting to our new size, so it should level out with time.
Or it could just be that you're stressed and you need some personal chill time. If that's the case.... spa day, anyone?