Our cat Smudge was 13 years old when we found a lump on her knee. Unfortunately it turned out to be cancer & she was sadly put down. I'm not saying that is what your cat has got, but you should really take her to the vet ASAP to get it checked. It may be nothing more than a cyst or something treatable. Try not to worry until you get there.
My cat is around 14 and she has several little lumps under her fur. The vet says they are just fatty deposits and are normal for her age. If the lump moves about easily and doesn't seem to be anchored to her body, then it's likely to be a fatty lump but it's probably worth a trip to the vet for your peace of mind.