Hi,
Scanty flow during periods is called 
hypomenorrhea. It can occur due to different causes like,
-Psychological factors like stress and strain affecting the reproductive hormones
-Weight changes due to changes in exercise, diet or sleep pattern, etc. also can affect the reproductive hormones and thereby menstrual flow
-General causes like anemia, thyroid disorders, serum 
prolactin levels alteration etc.
-Poly cystic ovary disease, etc.
If the hypomenorrhea persists, better to consult your Gynecologist once, get examined and go for investigations like complete blood picture, blood sugar levels, thyroid profile, serum estrogen and 
progesterone levels, serum prolactin levels and 
ultrasound, etc.
With this work-up the possible cause of your problem can be identified and treatment can be taken accordingly.
General measures like taking regular, balanced diet, calorie burning exercises and maintaining normal 
Body Mass Index also help in maintaining the normal menstrual flow.
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr. Sree Gouri SR