Gave duphaston. Brown spotting. Why am I still uneasy with regard to pregnancy?

Hi, I am 26 years old. My regular cycle varies from 29 to 33 days. However, this month I don t have yet my period. My first day of last period was April 23. I had 5 home pregnancy test that says negative and a transvaginal ultrasound on my 36th day saying negative as well. My doctor gave me duphaston for 5 days but I still feel uneasy because this is the first time that I didn t have this long. Is there any possibility that I could be pregnant even if the tests taken says otherwise? I m really confused. Also I had slight brown spotting for 5 days now everytime I would go to urinate. Please help.