Depending on the length of your cicle and the day you ovulated, you can be pregnant, but there's not enough hormones for the test to detect yet. Or you're just not pregnant at all. I guess you can already take the test again, since it's been 3 days since you took the first one. If it's still negative and there's no period, try taking a blood test or seeing a doctor. But try not to stress out, it could affect your period even more.