Basanti, a 28 yrs aged female with a history of 6 weeks of amenorrhoea presents with pain in abdomen; USG shows fluid in pouch of douglas. Aspiration yields dark color blood that fails to clot.Most probable diagnosis is:
The picture given in the question classically represents a case of ruptured ectopics pregnancy.
Symptoms – In Ectopic pregnancy triad of:

Amenorrhea (seen in 75% cases) followed by:
Abdominal pain (seen in 100% cases, it is the most consistent symptom of ectopic pregnancy).
Appearance of vaginal bleeding are seen:
The above triad may be accompanied by nausea, vomiting, fainting attacks or syncope.
Patient may present in shock with pallor, tachycardia, hypotension and cold clammy extremities, if ectopic pregnancy has ruptured

Examination:
General examination: In case of rupture—

P/R ↑
Pallor +nt
BP ↓
Slight intermittent pyrexia due to absorption of products of degeneration.

Per Abdomen - Abdomen is tense, tender and distended.

Shifting dullness may be present (depending on the amount of hemorrhage in ruptured ectopic).
Rigidity/muscle guarding +/–,
Cullen’s sign - bluish discolouration around the umbilicus may be present.

On Bimanual examination

Vaginal mucosa appears blanched.
Uterus: normal size/slightly bulky.
Extreme tenderness on cervical movement
Fornices-tender (Remember - tenderness in pelvis is the most constant sign of Ectopic pregnancy).
U/L adnexal mass: is palpable in one third to half of patient.

Culdocentesis:

It is a simple technique used to identify hemoperitoneum.
Fluid is aspirated from cul-de-sac via posterior fornix with the help of a needle.
If non clotting blood is obtained, it is indicative of an intraperitoneal bleed and probably a ruptured ectopic.
Ruptured ovarian cyst
Ruptured ectopic pregnancy
Red degeneration of fibroid
Pelvic abscess