What Does Missed Period With Negative UPT Indicate?

I have had 1 positive pregnancy test but then 5 negative?? could I be pregnant or is there a chance it was a flase positive? I have at least misse done period but cant remember if I had a period end of December around Christmas as was so busy. I am very regular, every 28 days and had kept on top of my dates up until Christmas so I may have missed 2 periods but I have def missed jan.