How Will I Ever Know? (Pregnancy Scare)?

I am a 20-year-old female. I am on continuous birth control, because I have female issues that cause a lot of pain, so I never have a period anymore. The last time I had sex was over 1 month ago, and it was with my now ex-boyfriend. I recently started getting nauseous, betting back aches more than normal, and I've had tender breasts. I decided I should get a pregnancy test just to be safe. It came up negative. Now, my question is, if I am pregnant, how will I ever know for sure since I do not have a period? I know that's what most girls watch, but since I don't have one, I will never be able to tell by that. Could my birth control interfere with the pregnancy test results? Please give me some advice. I'm really worried. I just don't want to harm the baby if I am pregnant by continuing on the birth control.