Thanks for query,
Sex during menstrual flow does not give you any complication.
The scanty period is due to your endomerium preparation.
due to hormonal
surge & withdrawal you get your endometrial thickness during each cycle & it sheds in menstrual period.
Some times due to hormonal level it is thick & some times it is thin .so flow of period depend upon it.
Hormonal level is depends upon anxiety
so nothing to worry for it.