Hello and welcome to HCM,
, the blood and other tissue fragments are propelled out from the uterine cavity
by contraction of the muscle layer of the uterus
When undisturbed the uterine wall contracts spontaneously and the blood and tissue bits move out in greatest amount during first few days of the cycle followed by light discharge after that.
During intercourse, there is contraction of the genital tract right from vaginal wall to uterine wall.
The two processes go hand in hand and thus there is some change/ disturbance in the normal rhythmic flow of the menstrual blood.
There is nothing to worry.
But take care of sexually transmitted diseases.
These diseases can be transmitted even after using birtn control pills.
Thanks and take care
Dr Shailja P Wahal