I'm 23 years old. For the past two months I have had a one day period which was lighter than normal. Usely I last 5 days but for past two months I have a one day period I have taken three pregnancy test all saying negative. Could I be pregnant if not what

Hormonal imbalance. Menstrual irregularities are not uncommon and anovulatory cycle is a common cause. If you miss more than two periods it would be time to see your doctor. If you do not wish to be pregnant, use contraception all the time. Consult this site for more information on this topic. http://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/Pages/causes.aspx.