She could well have hurt it today and just stiffened up later. If that's the case, then she'll likely improve after a few days rest. If you think the lump is related to the limp, though, I'd have her in to see a vet as soon as you can. Do you think the lump could have been there before and you just hadn't found it, or do you think it's new?
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."