There are two things you need to do and you don't need a Vet for either one.
1) Get a thermometer if you do not already have one. Take her temperature Am and PM every day. Without seeing her walk or examining her, I suspect you could have a hoof abscess. This will usually elevate a temperature slightly as it is an infection.
2) Go out early in the morning and check the temperature of her hooves, all up and down both hind legs. You can almost always pinpoint any sore, infected or inflamed area early in the morning. I have gone out with a flashlight before dawn to check a horse's feet and legs many, many times.
A warm hoof is almost always a hoof abscess. Abscesses are probably the main reason that horses go 3 legged lame over night. They can be acute and show up as severe lameness in a day or two or can cause chronic lameness for several weeks before they decide to 'mature' and break. If she is walking with only the tip of her toe on the ground, it is usually an abscess toward the back of the hoof.
You never want to use antibiotics or Bute if you suspect a hoof abscess. These drugs only slow down the maturation of an abscess and make the horse lame for a longer period of time.
You do not want to use any counter-irritant, liniment or 'rub' before you know what is actually wrong. Sometimes it is the wrong thing to do and it always confuses the diagnosis at it also causes heat that can be mistaken for the problem when it is only the liniment creating it.
Again, I sound like a broken record, but everyone should have a good thermometer handy at all times. If a horse has anything wrong from a snotty nose to a cough to a lameness, you need to track its temperature.
If you only have one or two horses, you should know what those horses' vital signs are when they are resting and well. Know their usual temperature, heart rate and how often they normally breathe per minute. Then, if you suspect pain from colic or a sickness, you know immediately if something is really wrong. If you call a Vet, you will be able to tell him your horse's vital signs and other pertinent information like the color of his gums and how quickly pressure on his gums turns from a white spot back to pink. Know what purple gums look like. Know how to check a horse's digital pulse in his lower leg.
There is no excuse for anyone to be an uninformed horse owner and there is no reason for anyone to call a Vet without this information to give him. Knowing these things can either save a horse's life or save a $500.00 Vet bill.