the possibilities according to your history are,
-after termination of pregnancy
sometimes periods can become irregular,
-with increasing age means in perimenopausal group anovulation
-in conditions like poly cystic ovarian disease
anovlatory cycles can be seen,
so you can wait for few more days, if still you get negative results better to consult gynecologist undergo ultrasound
and hormonal evaluation which will possibly find out the cause.