They pick up a certain hormone that your body produces when you are pregnant. As everybody is different, people make different amounts of the hormone and sometimes the test just doesn't pick it up. But if the test does pick it up, ie: a positive result, its always accurate as you only make this hormone during pregnancy.
I'd say that the chances of you being pregnant are low, as you have had 2 negative tests, but you still cant rule it out 100%.
You also need to sort this out quite soon, as if you ARE pregnant, the further into the pregnancy you are, the less choices you will have, if you know what I mean.....