My dog used to be my calm buddy, now he’s suddenly anxious and barking nonstop

Over the past month, my usually chill golden retriever has become really anxious. He barks at everything - cars, other dogs, even shadows - and it's nonstop, especially in the evenings. We haven’t changed our routine much, and no new pets or people have come around. I tried more walks and extra playtime to tire him out, but it doesn’t seem to help. I’m worried something might be bothering him physically or emotionally. Has anyone else experienced a sudden shift like this in their dog’s behavior? What helped you figure out the cause or calm them back down? Also, any suggestions for managing this barking without stressing him out more would be amazing.