Thanks for query,
The menstrual flow consist of endometrial tissue with blood due to sheddying of prepared endometrium when it was not used for fertilised ovum.
Due to different level of hormonal
level in blood the thickness of endometrium varies this leads to different in consistency of flow.
The cramps you get is due to collected blood in Uterine cavityAs you have lighter periods you gets cramps.Once it cleared & you have heavier blood you have less cramps.