40day cycle. last period jan31. spotted jan28-31. feb15 blood pregnancy test negative. feb 17 still no period. why? help me understand?

Prob normal: So your last period started on january 28th...That makes your next cycle due at end of february. Having a light cycle can be normal. If you are not trying to conceive, i would recommend using a contraceptive to regulate your cycles and stop the worry about if you are pregnant.

Irregular periods: Irregular periods are one of the most common problems obgyns treat. If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Thyroid and pcos testing are warranted. There are many options available to help regulate the menstrual cycle. Schedule a visit.