Hello, First thing, you should undergo one home pregnancy test or blood test for beta HCG to confirm or exclude pregnancy status as you are sexually active long time. In exclusion of pregnancy, reasons for low menstrual flow are hormonal imbalance, stress/ anxiety, thyroid problem, wt gain / extreme loss, irregular eating habits,psychological factor etc. You may need to do some investigations to rule out causative factors through USG pelvis, thyroid profile & other hormone assay etc. Consult with your doctor and take suggestive measures. Avoid stress / anxiety, take healthy diet with iron supplements, eat more fruits & vegetables, control body weight and maintain genital hygiene properly. If you need further consultation with me, then you can visit http://www.healthcaremagic.com/doctors/dr-soumen-patra/63326. Take care yourself.