There are at least a few explanations for this situation. 
Firstly, as we all know, no tests are reliable in full 100%. Some of them may give you false negative or false positive results — it simply means that they may tell you that you're pregnant, even though you're not. That might be the reason why you see a negative result after you initially confirmed your pregnancy with a positive one.
Another possibility is that the pregnancy ended by itself. Unfortunately, most early gestations end up in miscarriage — natural pregnancy loss at this stage is very common. In the past, women wouldn't even know that they were pregnant, but nowadays, we can detect even extremely early pregnancies and learn about their course. Read more