What Could Cause Delay In Periods?

For about 2 to 3 months my period has been very funny n ive been feelin weird, the first month my period came a week late an stayed for 4 day and was very light, the next month a week last then last month but stayed for 7 days off an on and was even lighter, then this month it came 2 dayslater then last month and steal light!!...ive been feelin really really sick,ive been gettin all the pregant systems but ive been to the doctor they said im not prego i took a urin test n it says the same thing!!, im 19 my period has never been like this let alone my body!!...