you better go for blood test
as it can detect low levels of HCG also it will give accurate result,
if test comes positive you can follow with regular check ups,
if test comes negative you go for ultrasound
that can detect any problem in internal pelvic organs according to that treatment can be taken.
with your doctor's advice you can take tablets for withdrawal bleeding in case of negative pregnancy test,
don't worry most of the problems causing infertility