Anything can change your period, specially a new diet or stress (if you are stressing about being pregnant, or school... Whatever). And if you are eating different food, or taking something out of your diet (like the no carbs diets) that can mess up with your stomach too.
If you are really worry, like what mai7 said, ask for a blood test. Some of the home tests don't work for all woman (specially early on).
Hopefully you find out soon..