Hematocolpos is a medical condition in which the vagina is pooled with menstrual blood due to multiple factors leading to the blockage of menstrual blood flow. The medical definition of hematocolpos is an accumulation of blood within the vagina. It is often caused by the combination of menstruation with an imperforate hymen. It is sometimes seen in Robinow syndrome, uterus didelphys, or other vaginal anomalies. A related disorder is hematometra, where the uterus fills with menstrual blood. It presents after puberty as primary amenorrhoea, recurrent pelvic pain with a pelvic mass. This can be caused by a congenital stenosis of the cervix, or by a complication of a surgical treatment. Mucometrocolpos is the accumulation of mucous secretions behind an imperforate hymen. Mucometrocolpos can sometimes cause abdominal distention. Symptoms
On and off lower abdominal pain lasting more than a week 
Pain with cramping, with episodes of worse pain in between
Vomiting without blood or bile
Abdominal bloating and distention 
Constipation and changes in urine output 
Tender breasts 
Vaginal bleeding or discharge
Severe cyclic pelvic pain
Urinary retention

Causes
There are four possible causes of hematocolpos
Imperforate hymen: An imperforate hymen is a medical condition where the female is born with a hymen that spans the entire diameter of the vagina, with no opening to the outside. An imperforate hymen may be diagnosed at any age. However, when a girl hits puberty, this type of hymen blocks the blood from flowing out and the bloods pools in the vagina.
This may cause a sensation of mass or fullness in the lower abdomen, pain in the stomach and back, along with problems with urinating and bowel movements. Chemical exposure of the vaginal epithelia may lead to tissue damage (up to and including necrosis) and swelling (edema). The edematous blockage of the vagina results in identical symptomatology as the imperforate hymen. Cervical atresia is a relatively rare Mullerian duct anomaly of the female reproductive tract. It is associated with acute or chronic pain in the abdomen or pelvic pain along with other reproductive problems. A significant share of the women with cervical atresia have it since birth, that is, congenital cervical atresia. However, Cervical Atresia is distinct from other Mullerian duct anomalies. Vaginal atresia: Vaginal atresia is another congenital defect which results in the uterovaginal outflow tract obstruction. it occurs when the caudal portion of the vagina fails to form and is rather replaced with fibrous tissues only. Vaginal atresia has three basic categories of anomalies- vaginal agenesis, ambiguous genitalia, and imperforate anus and urogenital sinus variants. The different features associated with an ambiguous genitalia which might eventually lead to a blockage of menstrual blood flow are:  i) Rugal folds over the labia ii) Mass in an apparent labium iii) Excess clitoral tissue. Due to these anomalies, there are chances that the menstrual blood would not be able to flow out of the vagina, eventually leading to hematocolpos.