If you get a positive test result followed by a negative the same day, it could happen if you're using two different brands of pregnancy tests. A pregnancy test has a sensitivity level, and one might be more sensitive than others, which can change the test result. Also, the most accurate reading can be achieved when you do it with the first-morning urine as it's more concentrated with the hCG level. The later in the day you take the test, the more diluted your urine will be and change the test result. Other reasons could be that you took an expired pregnancy test kit, drank a lot of fluids before taking your first test, or misused the test. It could also be an early pregnancy loss in some cases. Early loss can be heartbreaking, and you should consult a doctor to deal with it.