It's likely that you tested too early. Most home pregnancy tests won't register a positive result until you've been pregnant for about two weeks. Here's why: Home pregnancy tests work by detecting the presence of the hormone human chorionic gonadotropin (hCG) in your urine. Your body starts to release hCG shortly after conception. If you got a negative result, it's possible that you could still be pregnant but weren't yet producing enough hCG to register a positive test result. Levels of hCG multiply very quickly early in pregnancy, so you should wait a couple of days and test again.
Another possibility is that you're having irregular periods for some reason. Things like gaining or losing weight or excess stress or exercise can throw your cycle off from month to month. Of course, these issues can also delay your ability to get pregnant, so you should talk to your doctor about ways you can alter your lifestyle to encourage conception.
If you don't get your period for several months and pregnancy tests are still negative, see your doctor to determine if you have a hormone imbalance that could be affecting your period, like thyroid disease or overproduction of the hormone prolactin. In either case, these conditions can be treated with medication and you should be able to get pregnant once they're in check.