When you are in PMS, you are experiencing hormonal changes, so that affects on the appetite as well. Before you have your period your appetite is increased (in most cases) and that's a classic symptom of PMS, and during the period it's decreased or normal. That's completely normal.
If you notice that things with your appetite are going out of control, talk to your doctor, because it can indicate some other problems. But, I think that it's just PMS-related and it's normal and it's nothing serious. If you have problems with you weight because of the increased appetite before your period, try to drink plenty of water and exercise.
Your levels of hormones increase and you would use alot of energy before your period because all that work the body prepard you for is drained and uses more of your energy and sugars which causes you to have an increase in appetite.
It happens to me too....