It's been raining for weeks, so I had no doubt my little horse was developing an abcess. But I couldn't see anything, and the lameness got worse, so after a week I had a vet look at it-- appears to be navicular disease! I'm devastated. She's actually sore in both feet, but because one is much worse, it's very hard to tell. We don't know what brought it on. She has a very light riding schedule.
I'm now looking to find a farrier who can either trim or shoe her (for awhile anyway) to get past the lameness. X-rays when the vet gets back from his holiday.
Just a reminder when trying to diagnos our horses--- we can be very, very wrong.