What Causes Delay In Periods?

Hello.. I'm a bit confused and annoyed at the same time... I always get my period on the 17th of each month, never been late if anything if it ever changes ( which is rare) it would come a few days earlier... So this month i waited for it and it arrived on the 22nd... I only got it for 3 days the first day it was pretty normal and the next 2 were surprisingly light... by the 3rd and last day i had nothing really there..... And i forgot to mentioned that i had nausea from b4 my period arrived till right this minute, everyday non stop.... A few days after my period was over I had a sharp pain from my belly button down to my clitoris.... felt like a pulling pain and i couldnt really make sudden movements.... It lasted that whole evening till the morning... The next morning i decided to go to the ER and try to figure it out .... they ran urine test and the preg test came out negative and he checked for any infections and i was clear... he took blood work and i was checked by a gyn and i was fine....... I'm clueless.... Im tired everyday and I have dizzy spells, Im nauseous all day long and i cant eat so much cause my stomach feels icky...... The doctor did mentioned that it could have been to early for the pregnancy test     .... What could be going on??