There could be many causes of irregular periods
( disease of the lining of the uterus ), uterine fibroids, ovarian cysts - all cause irregular periods.
Conditions like polycystic ovaries
Exercise, stress, weight loss
or gain and other medications plus diseases like thyroid problems, anemia, adrenal disorders also cause irregular periods.
You might have some local lesion on the cervix that causes such irregular bleeding.
is another common reason.
Since your pregnancy test at home is negative, please consult a gynecologist at the earliest, and go for a proper pelvic check up and ultrasound and other tests that she might suggest based on the findings.