my 4 year old border collie was fine alone for years and three weeks ago she suddenly started having full meltdowns and i cannot figure out what changed

I need outside eyes on this because i feel like im taking crazy pills. Juno is a 4.5 year old border collie, we have had her since she was 9 weeks. She has been alone in the house for 4-6 hour stretches without issue since she was about 14 months old. Crate trained as a puppy, transitioned to free roam at 2, totally normal. We have an indoor camera, we have years of footage of her napping on the couch while we are out.

Three weeks ago, with zero warning, she started losing her mind when we leave. Im talking pacing for 45 minutes, panting heavy, scratching at the back door (never done that in her life), one day she ripped a corner off a rug. Yesterday she had what looked like a full panic attack on camera and i had to leave a work thing to come home because i couldnt watch it. Nothing changed in our schedule, nothing changed in our house, no construction, no new pets, no new humans. My husband and i are both on the same routine we have been on for 18 months.

I have read every separation anxiety thread on the internet and every single one starts with "what changed" and i swear on my life nothing changed. We have not moved, no one died, no one had a baby, no one is gone more than usual. Vet appointment is monday but she does not present sick in any other way, eating fine, energy fine when we are home, no lameness, no obvious pain.

Has anyone had this happen completely out of the blue? Im not even sure what to ask the vet to look at because she is not symptomatic in any other way. The behavior is so specific and so sudden that it almost feels like something medical but i dont know what. Trying not to spiral.