Week late into her period, had sex once on her 21st day of cycle, and i pulled out, she is having watery white discharge instead of period since then?

Some thoughts...: In a woman with regular periods, some causes of a late period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (like birth control pills), hormonal imbalances, etc… Pregnancy is the common cause of a missed period, if sexually active. A home pregnancy test is very accurate when done 3 weeks after sex.