Hell. Thanks for choosing HCM.
With a many time negative urine pregnancy
test, there is no chance that you may be pregnanct.
There can be other causes of amenorrhoea, common amongst them being functional ovarian cysts, which will prevent bleeding. Other causes can be some hormonal
imbalances, infections or even an early menopause
(though you are too young and that is a remote possibility)
The ideal approach presently will be to get an abdominal USG which will reveal if there are any ovarian cysts.
Secondly you should go in for the following blood investigations:
1. Serum LH, FSH, PRL
2. serum Estrogen
Thirdly, your gyneac may prescribe some medicines for causing bleeding in case of estrogen progestrone imbalance.