9 days late and negative tests??

I'm 16 years old and my friend and I have been fwb for 8 months-- condoms are used every time. We haven't had sex in five weeks but I'm currently 9 days late. I have all the symptoms one would have before getting a period, and I'm so confused.. so I took two pregnancy tests yesterday and they both came back negative. Could the irregularity be due to my age?