If you're lucky! How thick the lining of the uterus gets in the couple of weeks before you get your period is what determines how light, heavy, long, or short it will be. Since most of us ovulate each month, the hormone levels that grow that lining are similar each month and so our periods are pretty consistent. Some months though, if you don't ovulate or if your hormone levels were lower for other reasons (like big weight loss, heavy exercise, big stresses in life), you may find you have a shorter or lighter period than normal. As long as your period is lasting about 3 or more days, chances are you're just fine. But if you normally have gushers and this one is a trickle, and you're sexually active (or anything close to it), be sure to do a pregnancy test! Sometimes, in early pregnancy, you can have a really light period, so if this period is really different than usual for you, don't ignore it. Some girls will have short and light periods every month and that's just who they are. Lucky them!