Some women experience mid cycle spotting for a couple of days and this might actually be a sign of ovulation. This is because just before ovulation occurs, there is a little dip in the level of hormones produced by the ovary. This results in slight spotting before the levels build up quickly again.
Some gynaecologists will prescribe the birth control pill for a few cycles to see if the mid cycle bleeding stops. The rationale is that if ovulation is the cause, then the birth control pill works by stopping ovulation and so there is no bleeding.
It is unusual to bleed continuously for 3 weeks though. Even for normal menstruation, it usually does not last beyond 1 week. In a young woman, this is quite likely due to hormonal changes related to delayed or missed ovulation. However, there can be other possibilities such as polyps, fibroids, inflammation or sometimes infection of the genital tract. If the prolonged bleeding is persistent, do see a doctor for an evaluation.
Best rgds
Dr YF Fong