I would first make a vet appt-anytime there's a big change in behavior a physical reason needs to be ruled out. This exact thing happened to a friend--she thought he was "just acting out" to punish her for being busy (dogs don't do things to punish us!). It turned out he had cancer and was dead within 3 months!
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Shannon & Puff 
"All his life he tried to be a good person. Many times, however, he failed; for after all he was only human. He wasn't a dog." Charles M. Schultz