What Causes A Delayed Period With Negative Pregnancy Test?

Hello Doc, For past 5 months my periods are irregular and days were 40,45,30,45,60. All the pregnancy test was negative, I am worried. What could be the reason for these later periods with pregnancy test as negative? Every month i expect the result to be positive, but i get disappointed every month. Please help me on my query. Note: Before marriage my period was regular with 30 days.