I vote abcess as well. These can be notorious -- just one day the horse shows up lame and no one can figure out why, then "poof!" the abcess exits and the horse is "magically" sound again.
Check her whole leg again, starting at the shoulder and firmly apply pressure all the way down. If no reaction, then use your fingers to apply more specific pressure all the way down and around. I'll bet there won't be any reaction. At the hoof, press firmly with your thumb around the coronet band. Feel the entire hoof for localized heat. Apply firm localized pressure on all areas of the sole and frog. Hopefully you will find a sensitive area. You may not have enough strength in your fingers to do this -- hence the hoof testers. You can also try tapping with the back of the hoof pick, but this is not as good to pinpoint a specific area. It may tell you for sure if the pain is in the hoof or not though.
I would also eliminate the anti-inflammatories until you know what the problem is. I never give meds unless I know what I'm giving them for. You can soak the foot in epsom salts. If there is an abcess, this might help bring it out, softening the hoof. If there isn't, it won't do any harm.