How can i help my cocker spaniel ??? What is it she's suffering from?

My cocker spaniel is 15 years old and is a senior she over the past 2 weeks has been limping and has not been very healthy she struggles to walk or even stand up. We don't know how but all of a sudden she had like a tumor or something it is really hard and it's the size of a golf ball. It's located near her stomach but lower. Eventually This infection passed on to her leg and now it's swollen and it hurts her causing her to limp . Please help me in any ideas you think it is or how we can help her . I'll appreciate it :D. Thanks