Petechiae or Purpura: Given the sudden onset of small, purple spots (about 4 mm in size) on the ankle without itching but with some pain, petechiae or purpura are strong considerations. These conditions can be related to minor trauma, vitamin deficiencies, or medication side effects, which might be more common in a patient with a history of fibromyalgia who may be on various medications.

Vasculitis: This could present with purpuric lesions, especially if the patient has an underlying autoimmune condition or is on certain medications. The pain associated with the lesions could indicate inflammation of the blood vessels.
Thrombocytopenia: Although not directly indicated by the symptoms, if the patient has a low platelet count due to medication (e.g., certain drugs for fibromyalgia or hypertension) or another condition, petechiae could be a manifestation.
Skin Manifestation of Fibromyalgia: Some patients with fibromyalgia may experience skin manifestations, including sensitivity or reactions that could potentially look like purple spots, though this is less common.

Deep Vein Thrombosis (DVT): While the description doesn't classically fit DVT, any sudden onset of pain and discoloration in the leg should prompt consideration of DVT, especially in a patient with risk factors like hypertension and possibly decreased mobility due to fibromyalgia.
Meningococcemia: This is a severe infection that can cause purpura, but it usually presents with more systemic symptoms like fever and is less likely without such symptoms. However, it's crucial not to miss this diagnosis due to its high mortality rate.
Disseminated Intravascular Coagulation (DIC): This condition can cause purpura and petechiae among other symptoms and is a medical emergency. It might be considered if there were other signs of coagulopathy or if the patient's condition rapidly deteriorates.

Henoch-Schönlein Purpura (HSP): A form of vasculitis that can cause purpura, but it's more common in children and often associated with abdominal pain, joint pain, and renal involvement.
Amyloidosis: Can cause purpura, especially when associated with certain types of amyloid deposits, but this would be an unusual initial presentation without other systemic symptoms.
Scurvy: Vitamin C deficiency can cause petechiae and purpura, but this is rare in developed countries without a highly restricted diet or specific malabsorptive conditions.