How do tell if my horse has an abscess?

My horse has been limping for almost two weeks and I can't find anything wrong except when I cleaned her hooves today, in her left front hoof, there was something hard next to her frog. I thought it was a rock so I tried to get it out, but then I realized it was part of her hoof. I scraped it off with the hoof pick with some difficulty and inside it was really white and kinda powdery and flaky-like. Is this an abscess???