性交後的月經

我唸你應該係處女膜穿左！

正常處女膜穿左會有少少血流，但係唔會好似來M咁流咁多，一陣間就止到血架喇…

如果你真係有左，唔會流血架，流血即係代表你小產= ="同埋都唔會咁快知道係咪有左，最快都要2個星期先驗到！

if you had sex on 15th, 16th and 17th; and your 處女膜 was broken on those days, you shouldn't be bleeding on the 21st. When your 處女膜 is broken, you should be bleeding (just a small amount) on the when the breakage happens (either during sexual intercourse or exercise).

NOTE: various exercises can also cause the 處女膜 to be broken prior to first sexual intercourse.

Although I am certain that it cannot be your 處女膜 due to the huge time delay (4 days), it may be that your virginal wall was slightly damage during sexual intercourse (i.e. not enough secretions during sex..etc.) and the blood has flown out. It would be a vary slight/small damage because:
A) you didn't bleed during sex
B) your virginal wall wounds has healed already cause you've only experienced slight bleeding couple of days afterwards
C) you didn't feel any unusual pain after sex or during sex

我有時都會1個月2次既
但係唔係個個月都係囉
我未性交過所以唔係好知