Yes, it is possible to be pregnant even though you have had a negative pregnancy test, so you should definitely consult your primary care doctor
or OB GYN to be sure! Pregnancy tests will often cite a 99% prediction ratio (meaning at least 1% are wrong, right?), but those numbers are based on some assumptions. First, if your test is taken before you would normally have had your menstrual cycle, it is possible that the hormone levels are not high enough. Also, if your cycle is irregular, that can make it difficult to predict at what point you did get pregnant, and can thus throw off a pregnancy test. For most people, however, who are following the instructions on the box or are at the time of their next period, these tests are highly accurate.
The test that your doctor can do in his or her office is much more accurate at determining if you are pregnant, and if you have concerns, you should speak with your doctor to determine if you should be tested in a more accurate matter. Regardless, if you are a female of child bearing age, you should use proper contraception
(and protection!) or consider appropriate prenatal care, such as a multivitamin or other healthy steps such as avoiding alcohol or tobacco smoke. Please book an appointment with your doctor immediately!