Hello,
A complete clinical examination is very important for correct diagnosis and treatment of your condition. There can be many causes of your decreased menstrual flow.Stress,anxiety,excessive exercise all can suppress the activity of the centers in the brain that stimulate the ovaries during the ovarian cycle resulting in low production of these hormones. Since pregnancy is ruled out other possibilities should be considered. You need a clinical examination and tests like hormonal assay, pelvic ultrasound .That will be helpful. Treatment will depend on the underlying cause. Keep me posted.
Best luck and regards!