It’s impossible for anyone to guess whether it could be aggravation of an older injury or something new entirely. In theory, it could be anything.
I agree with those who say that it sounds like an abscess. Suddenly not wanting to weight the heel (or other part of the foot) is usually a pretty good indication of that. Also not surprised to hear of the ebb and flow of pain with bute - although abscesses can go through the same patterns without bute.
I’ve had many instances where horses don’t react to hoof testers, have no digital pulse, no heat, and the lameness comes and goes. I’ve also had the swelling aspect run the gamut from none to stovepipe leg. On a side note, I’m surprised that your vet told you to bute it - I too have been told (by my vet) to never bute an abscess as it can cause it to take longer to come to a head.
The last abscess I dealt with was with my gelding who hasn’t had an abscess in the last 5 or 6 years. It was a front foot and it was very clear that he didn’t want to weight the lateral heel portion of his foot. He was dead lame when that foot was on the outside of a circle, totally normal looking when it was on the inside. My vet was convinced that it was a soft tissue injury. I was convinced it was not (mostly because of the timing in that it started the day after he was reshod - and there was zero swelling in his very-visible-leg-structures). The lameness went on for about 3 weeks and we ended up x-raying his foot and scheduling an ultrasound for his leg (which we never ended up doing). During that time I also had my farrier back out to pull the shoe and take a closer look at his foot (no signs of an abscess or anything “odd”, also no major reaction to hoof testers).
After about 3 weeks of slowing-resolving-lameness he returned to maybe 98% soundness (there would be a tiny bit of a bobble when he turned a sharp corner where that foot was on the outside). By 5 weeks he felt completely normal. When he was reshod at 6 weeks, there was a massive abscess track that started in his lateral heel and tracked through his frog, exiting on the inside edge of his frog. He hadn’t reacted to hoof testers at all in that area, though his lameness certainly corresponded to where the abscess was.
Based on what you’ve said, I would still guess abscess and I’d be wrapping/soaking his foot. Abscesses don’t typically come and go with exercise, but it sounds like yours has come and gone with bute, not necessarily exercise, and that doesn’t sound unusual.
