What Causes Delay In Period?

I am 2 weeks late for my period, which were regular 28-30 days.
I did 2 Urine tests during the 1st week of being late, both negative. Went on with my daily duties, until 4 days ago I started becoming really tired, urinating a lot, feeling sick every evening and for the past 2 days have had really heavy, tender breasts.
I know I am showing all pregnancy signs so I took another test this morning, negative again. What is happening?
Could I still be pregnant?
Or is something bad happening?