Symphysis Pubis Dysfunction (SPD): This condition is characterized by sharp pains in the vulvar and pelvic area, often exacerbated by walking or movement. It's a common issue in pregnancy due to the relaxation of pelvic ligaments.

Vulvar Varicosities: Similar to varicose veins in the legs, vulvar varicosities can cause pain, especially after standing or walking for long periods. Pregnancy increases the risk due to increased blood volume and pressure on veins.
Pudendal Neuralgia: This condition involves pain in the distribution area of the pudendal nerve, which includes the vulva. It can be exacerbated by activities like walking and is more common in pregnancy due to the pressure on the nerve.

Deep Vein Thrombosis (DVT): Although less common, DVT can present with pain in the vulvar area if the clot is located in the pelvic veins. Pregnancy increases the risk of DVT, making it crucial not to miss this potentially life-threatening condition.
Ectopic Pregnancy: Although the patient is 18 weeks pregnant, an ectopic pregnancy can still present with sharp, episodic pain, especially if there's a rupture. This is a medical emergency.
Placental Abruption: This condition involves the placenta separating from the uterus and can cause severe pain. It's a serious condition that requires immediate medical attention.

Vulvar Dermatological Conditions: Certain skin conditions like lichen sclerosis or psoriasis can cause pain and discomfort in the vulvar area. These are less likely but should be considered if other diagnoses are ruled out.
Neurological Conditions: Conditions like multiple sclerosis or peripheral neuropathy can cause vulvar pain, though these would be rare in the context of new onset during pregnancy and related to walking.