Lex, you need to call your doctor, of course.

Here are a few of the most common possibilities, but only your doc will know for sure.

Some common causes in women of reproductive age are the birth control pill, which can cause bleeding between periods; problems with hormones produced by the thyroid, ovaries, pituitary, or adrenal glands; infections, such as sexually transmitted diseases, of the vagina, cervix, uterus, fallopian tubes, or ovaries; and miscarriage or problems with pregnancy.

Low-dose birth control pills may cause hypoplasia—a too-thin uterine lining—which in turn may cause bleeding after sex. Missing a pill or taking one late may also trigger this bleeding.

Hormones can interact with normal genital bacteria to create cervical irritation (cervicitis). Cervicitis is the most common cause of bleeding after intercourse and may frequently cause spotting, not just after intercourse. It's generally not dangerous and it can be treated with antibiotic creams. (Antibiotics can also clear up vaginitis, which sometimes causes bleeding.)

Chlamydia is a common sexually transmitted disease (STD)
Symptoms of chlamydia may include:

An unusual vaginal discharge
Bleeding after intercourse
Bleeding between menstrual periods
Abdominal or pelvic pain

Endocervical polyps—benign grapelike growths that protrude through the cervix—may cause bleeding when they're jostled during lovemaking.