What's wrong with my rat?

My rat is a little over 3 years old now. She's showing all of the signs of old age, so I knew that she didn't really have a lot of time left. But then this big bulgey thing is growing on her left leg..well, not ON her left leg, but her lower stomach near her back leg. It keeps growing, and its got this blueish color (the rest of her skin is pink), and now instead of one bulge, they're more like two bulges really close together. They're really REALLY big (they take up about a quarter of her body together), so she can't really use that leg anymore, and every time she walks, she's sliding around on that bulge, so that probably doesn't feel good. She's also not using her other back leg, so now she's scooting around with her front paws. AND ever since she got that lump, she's been losing her balance when she walks and sometimes completely flips over onto her side. I thought the lump was a tumor at first, but it's not like a hard lump. It's just really really squishy, and it's been there for a while. I'm kind of wondering if there's some kind of fluid building up there... what's wrong with her? And should I take her to the vet to get euthanized? I don't want her to suffer for a long time, and my parents WILL NOT get the procedure done to fix her because it would probably be expensive, and they're too cheap. PLEASE I NEED HEEEEEEEELLLLLLPPPP!!!!!