our 5 month old golden was completely reliable in the house for 6 weeks and last sunday everything fell apart, three accidents in two days, looking for help thinking through whether this is medical or behavioral

Bramble is our first dog and he is a 5 month old golden retriever we picked up at 9 weeks. we did the standard potty training protocol from the breeder, crate at night, out every 90 minutes during the day for the first month, reward at the exact spot, etc. by 14 weeks he was telling us at the door and we genuinely had not had an accident in 6 weeks. i had let myself relax about it, was telling my mother in law on facetime that we got lucky with an easy puppy. cosmic karma punished me immediately.

What happened. sunday morning he peed on the kitchen rug while i was making coffee, 8 feet from the back door he normally asks at. he did not give me any signal. i caught him mid stream, said nope, took him outside, he finished outside, treats and praise like normal. monday afternoon he peed in the front hall right after coming back in from a 15 minute pee break outside where he peed twice. i was actively watching him, he just squatted and went. monday night he pooped in his crate during the 4 hours we were out at dinner, which he has never done in his life. i cleaned everything with enzymatic cleaner, scrubbed the rug, the works. he seems totally fine personality wise, eating normally, playing normally, no obvious sign of being sick.

What i have been wondering. is this a 5 month old hitting some kind of adolescent regression phase. is this a UTI or something medical i should be calling the vet about tomorrow. is this me getting complacent on supervision and giving him too much freedom too fast (we did expand his access to the kitchen and front hall in the last 10 days, before that he was on a leash to me indoors at all times). is this something with his food, we switched bags two weeks ago from his puppy formula to the next size up of the same brand.

Specific questions for anyone who has been here. how do you actually tell the difference between a medical regression and a behavioral regression at this age without dropping 400 dollars on diagnostics every time he has an accident. if you went through this, what was your dog actually doing and what fixed it. and is it normal at 5 months for the wheels to come off a little even on dogs who seemed solid, or did i miss earlier signs that i was just lucky to get away with for 6 weeks. i feel like a brand new puppy owner again and i thought we were past this part