No birth control is 100% effective, so see a doctor to run a test for you. If you've had unprotected sex at all, there will always be a chance you could be pregnant. There are women that are pregnant and will experience some menstrual blood flow, so see your doctor to have them check things out.
Over the past 3 months, I've taken 6 pregnancy tests and all have come back negative. I've had my periods but at times I've felt slightly nauseous and queasy. What is the probability that I am pregnant?
In addition to Danae's excellent answer.
You should use reliable birth control.
Pregnancy isn't the only thing that can make you queasy or nauseous. If you are having this this problem frequently, you need to talk to your doctor. It could be something as simple as a food intolerance or something more serious. Unless you go to the doctor you will never know. While you are there talk to your doctor about birth control. If you are already using it, that maybe the problem and you may need to switch.