I will echo what everyone is saying already: you need to eat some more. Hunger is a good indicator that you need the calories, as lph is pointing out. Based on my own "experiments" with long bike rides, I have learned you need to eat at the right times, too: if I do not eat after a long ride (read 4+ hours) I will be ravenous later on.

Ask your doctor for a thyroid check (it's a simple blood test). And, let the fit in clothing be your guide, rather than weight. Muscle is more dense than fat, so you can gain weight and your clothes can get looser.