What Causes Delay In The Menstruation Cycle?

Hi doctor... i am 24 yrs old female, its 9months after my marriage.. from january v started trying for our baby.. v had intercourse on jan 20, 24, 30. my last menses was on jan 4th.. till now i didnt get my menses.. but mymenses is irregular after marriage.. before it was 30 days cycle.. i consulted doctor abt my late menses did ultrasound to check whether i have PCOD.. but everything was normal.. i did home pregnancy test but it came negative.. so wat will be the reason?