I go through this a couple of times a year and it seems to happen for different reasons - at least for me. One is my body is fighting something off and it's like I have the "almost flu." Nothing sounds good and I basically have to force myself to eat anything. Two is I am stressed and don't even realize it until later. Three is sometimes I feel like I am in an "anti-growing spurt" - kind of the opposite of when my kids seem to be in a growing spurt and eat everything in sight! Again, I end up forcing myself to eat.
I would certainly check it out with your doctor to make sure that nothing IS wrong, but if he/she medically clears you, I wouldn't worry too much about it. In fact, for me, the more I obsess about how much I haven't eaten that day, the less hungry I am. What I do though is eat every time I AM hungry. Even if it is at 1:00 a.m. in the morning. I keep lots of easy stuff in the house when I go through these periods so the second I am hungry I have something available because often if I have to go through any effort to make it, I am done being hungry by the time it is ready :)
Good luck.