Can polymyositis be really painful?

Mildly painful. Polymyositis is a persistent inflammatory muscle disease that causes muscle weakness. The signs and symptoms develop gradually. The hallmark is progressive muscle weakness, additional symptoms can include fatigue, mild joint and or muscle pain, difficulty swallowing and shortness of breath. The pain is usually mild.
Usually some aching. But not terribly painful! More like generalized bruising.1.
No likely. Polymyositis is classically described as painless weakness. While mild discomfort with activity due to the weakness can occur, it is usually not very prominent. Severe pain should make one look for a different diagnosis.