What Causes Irregular Periods?

I took Clomid and had HCG Injection in january, my period due date was 14/02 but i started 12/02 where i had normal flow bleeding but did not last the whole day, basically i had half day bleeding only. My period stopped there, next day only brown stain on tissue few times and finished. I had a half day period, what does that mean. Few hours before i started my period i did a early pregnancy test and it was negative.