Erythema nodosum: This condition is characterized by the sudden onset of tender, erythematous nodules or plaques, typically on the lower extremities. It can be idiopathic or associated with various conditions such as infections, medications, or systemic diseases. The lack of response to antibiotics or steroids and the absence of DVT on US make this a plausible diagnosis, given its inflammatory nature and potential for recurrence.

Cellulitis: Although the patient did not respond to antibiotics, cellulitis can sometimes be resistant to initial antibiotic treatments, especially if the causative organism is not adequately covered. Re-evaluation of the antibiotic choice or consideration of other infectious causes might be necessary.
Stasis dermatitis: This condition occurs due to poor circulation and can cause redness, swelling, and pain in the lower extremities. It might not fully respond to antibiotics or steroids if the underlying circulatory issues are not addressed.
Contact dermatitis: If the patient has been exposed to an allergen or irritant that affects both lower extremities, this could be a possible diagnosis. The lack of response to antibiotics or steroids would be expected, as these treatments are not effective for contact dermatitis.

Necrotizing fasciitis: Although rare, this is a life-threatening condition that requires immediate surgical intervention. The presence of severe pain out of proportion to the physical findings, fever, and signs of systemic toxicity should raise suspicion for this diagnosis.
Vasculitis: Certain types of vasculitis, such as polyarteritis nodosa or granulomatosis with polyangiitis, can present with skin manifestations including redness, pain, and swelling in the lower extremities. These conditions can be severe and require prompt diagnosis and treatment to prevent organ damage.
Deep vein thrombosis (DVT) with negative US: Although the ultrasound was negative, DVT cannot be completely ruled out, especially if there are strong clinical suspicions. A negative D-dimer or further imaging with CT or MRI venography might be considered in high-risk patients.

Chronic lymphangitis: This condition involves inflammation of the lymphatic vessels and can cause redness, swelling, and pain in the affected limb. It's less common and might be considered if other diagnoses are ruled out.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the lower extremities, leading to swelling, pain, and bruising. It does not typically respond to antibiotics or steroids and requires specific management strategies.
Panniculitis: Inflammation of the subcutaneous fat, which can cause painful nodules or plaques on the lower extremities. It has various causes, including autoimmune diseases, and might not respond to standard antibiotic or steroid treatments.