Given her age, I wouldn't be surprised if she has arthritis. Our old mare had a big bump on her knee and the vet confirmed it was caused by arthritis. It didn't cause her a lot of pain, and the vet told us it was fairly common in senior horses. Basically, the joint fluid starts to leak out and creates a lump. The vet did say we could drain it, but the risk of infection was greater than the benefit of drainage, since it would just come back anyway, so we left it.
Definitely have her examined by your vet first to make sure she is suitable for light riding.