Why am I bleeding after messing around?

Many people have misconceptions about the “cherry” or the hymen.

The hymen does not magically disappear when something is inserted into the vagina, it will only stretch or tear sufficiently to permit entry of whatever is being inserted. If for example, a teen inserts two fingers into her vagina while masturbating, her hymen may still tear when she has vaginal intercourse for the first time, since the average penis is larger than her two fingers. A woman who has had vaginal intercourse may still have hymeneal tissue present; this remaining tissue can be the cause of pain during intercourse. If a woman’s current partner has a larger penis than her prior partners, or a couple tries a new technique or position during intercourse, her hymen may tear again, or for the first time. When doctors examine pre-adolescent and adolescent girls for evidence of sexual abuse, they look for injuries to the hymen; the hymen may still be intact except for a single tear. Remnants of the hymen are usually present until a woman delivers a baby vaginally.

more: http://the-clitoris.com/n_html/hymen.htm