There can be multiple causes of irregular and light periods
Pregnancy
Anxiety/ Physical or mental stress/ Anorexia/ Sudden weight loss
Obesity
Use of Ocps (contraceptive pills)
Anaemia/ Malnutrition
PCOS
Pelvic infection/tubercular endometritis
Synechiae (intrauterine adhesions)
Others - diabeties,
thyroid, medication, premature ovarian failure, tumors etc
Get yourself investigated to rule out all these causes.