Other than being pregnant there are a number of answers to your question. You period may, or may not, have been consistent in the past, but that doesn't mean it will be forever. Chemical changes in the body can make predicting your period difficult, you may be able to count on it for years and then a sudden change may occur.
Taking the pill, or the morning after pill, could effect your period significantly. Stress can also be a significant factor. One thing is for sure, missing your period is not a effective way of determining if you're pregnant. Go see your doctor, or get a home pregnancy test to be sure (you'll have to wait until your period is at least seven days late), or you can always wait and see what pops up, or out. Don't worry, or get excited, about it until a significant amount of time has passed. Being three or four days late could be caused by quite a number of things.
If you're concerned that your period is often late, or significantly late, the best idea is to tell your doctor and see what he says.
