There may be the following reasons for a dog to suddenly scream while sleeping well:

The dog Maybe it was a dream. Dogs sometimes have nightmares that cause them to wake up suddenly and scream.

Dogs may be insecure. When dogs feel insecure, they may suddenly wake up and scream.

The dog may be feeling unwell. Dogs with physical pain or other uncomfortable symptoms, such as heart disease, bone and joint disease, etc., may cause them to wake up suddenly and scream.

If your dog frequently screams during sleep, it is recommended to consult a doctor in time to see if a more comprehensive examination is needed.