Post-viral myositis or myalgia: Given the patient's recent viral infection, which could have been COVID-19, and the absence of trauma or previous back pain, a post-viral condition affecting the muscles could explain the pain and weakness. The fact that rheumatic causes have been ruled out and there's no indication of neurological deficits (normal reflexes, no tingling or numbness) supports this consideration.

Thoracic outlet syndrome (TOS): Although the patient does not have the typical symptoms of tingling or numbness, TOS could still be a consideration given the shoulder weakness and upper back pain. The absence of trauma does not rule out TOS, as it can be caused by repetitive strain or poor posture.
Myofascial pain syndrome: This condition could explain the localized pain and weakness, especially if the patient has been experiencing stress or has poor posture that could lead to muscle strain in the upper back and shoulder area.
Inflammatory myopathy: Although rheumatic causes have been eliminated, an inflammatory myopathy not related to rheumatologic diseases could still be a possibility, especially given the recent viral infection which could potentially trigger an autoimmune response.

Spinal cord tumor or metastasis: Despite the negative PET scan, it's crucial not to miss a spinal cord tumor or metastasis, as these conditions can have devastating consequences if not treated promptly. The absence of typical neurological deficits does not entirely rule out these possibilities, especially if the tumor is small or not yet causing significant compression.
Epidural abscess: This is a medical emergency that requires immediate attention. Although the patient does not have fever or other systemic symptoms mentioned, the possibility of an epidural abscess should be considered, especially given the severity of the pain and the presence of weakness.
Multiple myeloma: Given the patient's age is not specified, but considering the back pain and recent viral infection, multiple myeloma could be a consideration, especially if the PET scan was negative for metastases but did not specifically rule out myeloma.

Polymyalgia rheumatica (PMR): Although the patient has been evaluated by a rheumatologist, PMR could still be considered, especially given the shoulder weakness and upper back pain. However, the typical age of onset and the absence of other systemic symptoms (like fever or weight loss) make this less likely.
Dermatomyositis: This condition is characterized by skin manifestations and muscle weakness. If the patient has any skin changes, this could be a consideration, although it seems less likely given the information provided.
Neurosarcoidosis: This rare condition involves sarcoidosis affecting the nervous system and could potentially cause the symptoms described, although it would be unusual without other systemic symptoms of sarcoidosis.