My first thought is that you just had a wonky period that was lighter than usual. But if this kind of thing isn't typical for you, I wonder if you could be pregnant, but you tested too early, or if you have your ovulation dates off.
If you've tested within the last day, hold off on testing again. If it's been a few days since you last took a test, take one now. If it's still negative, I would wait until your next period is due to see if it comes and if that one doesn't come, call up your doctor.
It's possible you're pregnant, but the test is, for some reason, not giving you an accurate result.
It's also possible that you're dealing with an undiagnosed infection or treatable condition that only an OB could properly diagnose.
Keep in mind that's it's not unusual for women to skip a period or experience delayed periods once in a while, but if the spotting happens again without a menstrual flow, you'll want to call your doctor.