What could be reason for pain & tenderness in various muscles?

Any exercise: To be sore after exercise, especially if strenuous, is normal. If there is no exercise associated with it and if there is stiffness and is more in the morning would see a doc to rule out a rheumatological disorder.

Vague but will try : Viral infection (cold/flu), strenuous exercise, recent injury, certain medications or exposures known to cause myalgia/myositis or rhabdo, connective tissue disorders (e.g. Inflammatory myopathies, lupus), and a long list of other tings which can be contemplated after a better history is obtained.