No photos? Sounds like shin splints to me. She's young, it's on the cannon bone, they can cause lameness or not sometimes. It's a sign of overwork in young horses, or can be a manifestation of poor conformation.Sometimes they pop them on their own at play in the pasture, but pasture raised horses aren't as likely to develop it, in my experience. It's the stall bound colts that get longed a lot that tend to do it more. I would stop the "sweat" and cold hose/ice the areas. Give her a good rest, with turnout or reduce her work to slower gaits for a while. She'll be fine, but her legs may be permanantly blemished. Of course, I'm just guessing based on what you described with no pics or hands on exam.
Here's a link to wikipedia about it. http://en.wikipedia.org/wiki/Splints