What Causes Abnormal Menstrual Periods?

hi for the past 4 weeks ive been feeling sick and being sick whenever i eat anything im also very tired all the time i have done a pregnancy test which came back negative also my period was different i only came on for 2 and half days and it was really dark have you any ideas what this could be thanks i am 29 yrs old 4"11 weigh 14st