There are a few things which come to mind:
She could have a quicked nail - removing the shoe would determine if the nail is quicked or not.
She could have had more damage from the founder than realized and her soles may be thin - radiographs and a vet exam would help this.
She may be arthritic and have ringbone, in which case the drastic change of hoof angles would aggravate the ringbone - radiographs again would be useful for diagnosis.
She could have an abscess - hoof testers will detect this.
Founder and long hooves both can cause internal damage to the hoof structure, as well as ringbone. An arthritic horse will respond well to the use of NSAIDs (Bute), while a quicked nail, abscess or sole bruise generally will not respond. If you give her 1g of bute twice a day (every 12 hours) for a few (2-3) days and she improves, I would definitely have a vet come out to do radiographs and determine a course of treatment. If she doesn't improve then a good farrier should be able to determine the cause of pain in the hoof, but will recommend use of a veterinarian if needed.
I am not a vet, but as a horse owner this is what I personally would do.
They say money doesn't buy happiness -- well happiness doesn't buy horses!