What Causes Late Periods, Spotting And Negative HPT?

Hi I'm 20 years old I'm sexually active and I have fairly regular periods... they are either on the 3rd-6th of every month... This month however I was a week late and just recently I started to get pink/brown spotting when i wipe.. I took a pregnancy test about 3 days ago and it was negative. Idk what is going on... Why am I late.. Could i be pregnant?