By light do you mean pale coloured blood or scanty menses? In either case the first and most common cause could be that you are Anemic. Check your haemoglobin. If low take corrective measures. If normal then go for USG (Sonography) followed by some specialised hormonal tests if required.

hi ! hormonal disturbances are now a days commonly seen in 9 out of 10 women, due to which either there is heavy flow,or very scanty discharge, pain here and there in the body, or white discharge, etc,etc many face such troubles, in your case it is scanty discharge, where the menses appear but the bleeding is very much less. consult a good homoeopath in your vicinity and go for medication you will be fine with in few mnths and you will have a nrml period.