In short, you shouldn't be worried.
That being said, you shouldn't only eat when you're hungry. For example, dehydration is a very serious problem , especially in areas with hot/dry climate. However, many people who are experiencing dehydration do not feel the urge to drink water. That is completely normal, and is nothing to worry about IF--stress on the conditional--the person is proactive in drinking water even when he/she is not thirsty. The best way to do this is to keep a bottle of water around and take small sips throughout the day.
Sorry for the drawn out reply. The point is that "not feeling hungry" in and of itself isn't much to worry about. It also depends on how active you are. I tend to get more of an appetite if I'm active, and less hungry during the summer when I'm being lazy and sluggish.
The human body can survive on surprisingly little food. This is no reason, however, to deprive yourself. Eat small meals throughout the day, even if you are not hungry. You might not notice it now, but not eating as much as you should will lead to fatigue later on. Also important: get your H2O.
As a side note, I should mention that one would have cause to worry if this were to develop into anorexia or some other sort of eating disorder. Judging from context, though, this probably is not the case.