Why My Period Is Delayed Every Month?

Hi Sir, I got married recently and l miised my periods then i consulted doctor .Doctor conducted pregnance test and it was negative .Then doctor gave me some tablet to get periods. After haveing the tablets i got periods but this month also i didn't get the period and i did my pregnancy test this time also its negative.It would be great if you could provide me some details. I haven't lost any weight and no stress.