What Causes Delayed Periods While UPT Is Negative?

I had been on birth control for 10 years straight. I decided in September that I no longer wanted to take birth control and abbruptly stopped. Since September I have been having regular periods. I am sexually active with my fiance but he always wears a condom and it has never broken as we always check. I am now 4 days late for my period without any indication that it is coming (such as cramps). I took a pregnancy test today and it came back negative. I am worried it was a false negative as I have never been this late before. Could I be pregnant?