**Core Concept**
The patient's presentation suggests a severe case of postpartum hemorrhage (PPH), characterized by excessive uterine bleeding, leading to hypovolemic shock, which can cause symptoms such as amenorrhea, lower abdominal pain, facial pallor, and fainting.

**Why the Correct Answer is Right**
The likely cause of PPH is uterine atony, which is due to the failure of the uterus to contract effectively after delivery. This can be exacerbated by factors such as prolonged labor, multiple gestations, or retained placental tissue. The resulting blood loss leads to hypovolemia, causing a decrease in blood pressure, which in turn causes the patient's symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the patient's acute presentation of hypovolemic shock. While ectopic pregnancy can cause amenorrhea and abdominal pain, it does not typically present with shock.

**Option B:** This option is incorrect because it does not account for the patient's recent childbirth. Although pelvic inflammatory disease (PID) can cause lower abdominal pain and amenorrhea, it is not the most likely diagnosis given the patient's postpartum status.

**Option C:** This option is incorrect because it does not address the patient's acute symptoms of hypovolemic shock. While placenta previa is a cause of postpartum hemorrhage, it typically presents with painless vaginal bleeding, not the patient's symptoms of shock.

**Clinical Pearl / High-Yield Fact**
In cases of postpartum hemorrhage, it is essential to quickly identify the cause and administer uterotonic agents such as oxytocin or carboprost tromethamine to stimulate uterine contractions and reduce bleeding.

**Correct Answer:** C. Placenta previa.