Hematoma: Given the recent history of blood transfusion and ongoing GI blood loss, a hematoma is a plausible explanation for a red swollen area on the hip, possibly due to a bleeding complication or a traumatic injury during the transfusion process.

Cellulitis: A common skin infection that can cause redness, swelling, and warmth, often related to bacterial infection. The patient's age and potential compromised immune status due to blood loss and transfusion increase the risk.
Pressure Ulcer: In a 78-year-old patient, especially one who may have been bedridden or immobile for a period due to GI blood loss and transfusion, pressure ulcers (bed sores) are a concern. These can present as red, swollen areas, particularly over bony prominences like the hip.
Allergic Reaction: Although less common, an allergic reaction to the blood transfusion could potentially cause skin manifestations, including redness and swelling.

Deep Vein Thrombosis (DVT): While DVT typically presents with swelling, warmth, and discoloration of the affected limb, it is crucial not to miss this diagnosis due to its potential for leading to pulmonary embolism, a life-threatening condition. Immobility and recent surgery or transfusion increase the risk.
Necrotizing Fasciitis: A severe infection that can cause rapid progression of symptoms, including redness, swelling, and pain. It is less common but critical to identify early due to its high mortality rate if not promptly treated.
Septic Arthritis or Osteomyelitis: Infection of the joint or bone, respectively, which could present with localized redness, swelling, and pain. These conditions are medical emergencies, especially in an elderly patient with potential compromised immune status.

Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): A rare condition characterized by fever, neutrophilia, and tender, erythematous skin lesions. It can be associated with infections, inflammatory bowel disease, or malignancies, and could potentially be linked to the patient's GI blood loss or transfusion reaction.
Erythema Marginatum or Other Rheumatologic Conditions: Although rare, certain rheumatologic conditions could present with skin manifestations, including erythematous lesions. These would be less likely but should be considered if other diagnoses are ruled out and the patient has a relevant history or additional symptoms.