So Midnight is just over 19 weeks old. He had been doing very well with potty training up to about 5 days ago. I was watching another GSD for a friend who was 1 1/2 years old and after he left, Midnight started pottying (pooping and peeing) in the dining room where the other dog's kennel was kept while he was here. Midnight hadn't had an accident in the house for about a month before this and didn't have any in the house while the other dog was here. But 5 days ago, I started to find his accidents in the dining room even if we had just been outside 5 minutes before. Then last night, Midnight peed in his crate. He has never done this before and now I'm really concerned.
I've read on here that GSDs may start having accidents in the house when they start teething, but I'm not sure if that's what is going on. Do I just start over? Pretend I just brought him home and he needs to be trained from scratch? Is there anything else I should be doing?
Midnight is my first dog as an adult and the last GSD in my life was my brother's dog when I was 10-12. Advice?