I'm not trying to add to your worry, but I just wanted to add some knowledge which I learned from my dvd's that I ordered from Pete Ramey.
Someone was doing research on foundered horses and it may have been Dr. Robert Bowker, but I don't know who for sure. I would have to go find that segment. Loosie may know who it was exactly.
Anyhow. He was studying the seperation of the epidermal and dermal lamina from horses that were on feed that caused laminitis/founder. He would disect the hooves like a week after the horses were fed a diet that caused seperation. Then he would continue moving up the days and kept studying them sooner and sooner. I think it came down to where he could dissect a hoof something like an hour after they were given rich feed and he would find seperation.
I'm not all that great at conveying the details, but what I'm only trying to say is that laminitis can occur only hours after being on rich feed. It doesn't take lots of time for it to develop. I think there's a good article on it somewhere on the internet. I'll see if I can find it if I don't already have it saved.
Also I wouldn't really know if what you are describing is laminitis or not. It's possible though, especially considering that it's on all four hooves at the coronet band. But I honestly wouldn't know, though I also wouldn't rule it out. You can also educate yourself about when the best times to let your horse graze to help you with being proactive. Check out this site. www.safergrass.org
Basically the sugar levels in grass climb with drought and with the sun. So there is more sugar in grass at 6pm than there is at 6am because the sun has been out all day. And if the suns been out all week and it's been hot and there's been very little moisture the sugar levels are going to be way up too.