Thanks for posting on HCM,
There are many causes of secondary amenorrhea
with the most common being pregnancy
which you seem to have already ruled out with a pregnancy test but you could still be pregnant despite the test being negative. Try getting to a hospital so they can do a beta HCG
testing instead, if that is negative, then you are most likely not pregnant.
Other causes of secondary amenorrhea as you present, might be hormonal
imbalance, uterine abnormalities, stress, recent change of lifestyle/diet etc.
You will require to see a gynobs for physical, laboratory and imagery evaluation.
Hope this helps