My period has been regular for the last four months and now it's 17 days late and a couple days ago the pregnancy test was negative what's going on?

Menstrual: Irregularities are not uncommon. See this site for more info. If lack of periods persists for more than two cycles, please consult your doctor. http://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/pages/causes.aspx.

Period late: If you are at risk for pregnancy then that is still possible and perhaps the test is not positive because too early in the pregnancy, urine dilute, or rarely a test kit that does not work. It is also common to have a month when you did not ovulate and the normal trigger for the bleeding ( a drop in progesterone) does not happen. See a clinic for help.