I'm no vet, but if its in more than one leg, is recurring, and gets better with rest, then I'd be wary that perhaps its a chronic lameness brought on by the way in which the horse is constructed. The horse is such a complicated animal in the way it is built, and conformation that is not ideal in the legs could lead to the bones, joints, and ligaments/tendons becoming sore and swollen since horses spend almost their entire time on their feet.
I truly have no clue what it is, but I wish you luck. I think the best bet would be x-rays as soon as you can possibly get them, especially if this turns out to be a degenerative disease. That means that the quicker you get it figured out, the quicker you can treat and slow its progression. I understand the pressure of money and vet bills, as I am in my early twenties with horses...and yes, it is a big expense! As much as you may hate to, you may have to sell/give her to someone who can take care of her lameness issues. Particularly if it turns out to be something you're going to have to treat her entire life.
It could even be arthritis (I had a 4 year old QH with arthritis in his hocks, its strange but not unheard of for horses who aren't aged to have it. The only way we could tell that this is what he had was through x-rays.) His treatment was injections in his hocks once every few months. That adds up very quickly, and I ended up selling him to a family that wanted a pet they could ride very mildly every once in a blue moon. (He was more or less finished at 4 and had been a halter baby, which is most likely why he had develop the arthritis in the first place.)
I'm just throwing out possibities that are coming into my mind.
In any case, I hope that it isn't anything serious and can be treated easily so that you can keep your horse without having too much financial pressure! Good luck! Please keep us posted, I think we al here are anxious to know how it goes. ^_^