Get a vet to come Check her out. But if it's from age I would possibly consider either light riding or retirement.
Go see a vet! It can be serious especially with an older horse. If she's limping then there's something wrong with her muscles or bone probably. Until you do see a vet don't ride her because the situation can get worse. I hope she gets better! :)
I second calling a vet! Never be too confident in what you think is wrontg always question it and get lots of opinions.
I would definitely have your vet come take a look at it. Don't push it. Better to be safe than sorry.