the possible causes of your symptoms are,
-hypomenorrhea: means reduced blood flow during periods,
that can be due to many causes like anemia, stress and strain, weight loss
etc which will affect hypothalamo-pituitary-ovarian axis,
as urine pregnancy
test kits are less sensitive during early pregnancy you better go for blood test
for pregnancy which can detect low levels of HCG also and thus can rule out the possibility of pregnancy.