hello there..
here is what i think.. it could be:

Cervical dysplasia:
Cervical dysplasia is precancerous changes of the epithelial cells that line the cervix. Risk increases with multiple sexual partners, sex before age 18, childbirth before age 16, or a past history of STDs. Treatment is usually cryosurgery or conisation.

Chlamydia: A bacterial infection that is usually transmitted through sexual activity or contact with semen, vaginal fluid, or blood.

Gonorrhea: A usually sexually transmitted disease caused by a bacteria. Several pharmaceutical treatments are available.

Vaginitis or Cervicitis: Inflammation or swelling and infection of the vagina or cervix. Treatment depends on the cause.

Cervical polyps: Cervical polyps are smooth, red or purple, finger-like growths that grow out of the mucus layer of the cervix or the cervical canal. Cervical polyps are extremely fragile, extending out of the cervix, and easily and painlessly removed.

Trichomoniasis: A usually sexually transmitted disease caused by protozoan. Can also be passed to newborns during vaginal birth by infected mothers. Although rare, transmission is also possible in tap water, hot tubs, urine, on toilet seats, and in swimming pools. May cause vaginitis.

Vaginal yeast infection: An overgrowth of the normal fungi that inhabits the vaginal area. Common symptoms include itching, burning, and an odorless, white, cheese-like discharge.

Endometritis or adenomyosis: Endometritis is defined by Dorland's Medical Dictionary, 27th Edition as an inflammation of the endometrium (the innermost layer of the uterus). Both conditions are associated with endometriosis. Adenomysis is when endometrial tissue attaches itself to the uterus, or another organ such as the ovaries, and grows outside of the uterus.

Uterine polyps: Uterine polyps occur when the endometrium overgrows causing these protrusions into the uterus. It is extremely rare for these growths to grow in a way that is either benign or malignant. Women with uterine polyps frequently experience bleeding between periods (metrorrhagia), other symptoms includes vaginal bleeding after sex, spotting, menorrhagia, bleeding after menopause, and breakthrough bleeding during hormone therapy. Hysteroscopic-guided curettage is the preferred treatment, since the normal D&C is basically an unguided procedure that may miss many of the uterine polyps.

Fibroid tumors: Uterine fibroid tumors are usually benign tumors. They are solid masses made of fibrous tissue. Fibroid tumors are rarely malignant. Symptoms of fibroid tumors vary among women, with some women never experiencing any symptoms at all. Women who can wait until menopause will see their fibroids shrink and disappear once their bodies stop producing estrogen. It's important that women with fibroids make sure they never take estrogen, in any form including birth control pills, since estrogen increases fibroid growth. Several treatments are currently available for uterine fibroid tumors from myomectomy and uterine artery embolization to the traditional hysterectomy.

Diagnosing vaginal bleeding after sex is usually a matter of exclusion. Anytime you experience post-coital bleeding or vaginal bleeding after sex see your physician.