Seroma on cats back

Im new to this board. My cat is 16 yrs old. She has had a lump/ knot on her back several months. Recently it got larger and felt like it was soft and squishy. Being a nurse myself , i thought it was a lipoma. Anyway, went to vet last week, he said it was probably caused by her rabies vaccination given in June. I was shocked. The vet did labs, everything came back normal. She has lost weight but she is eating and drinking and going to the litterbox. He said that the seroma has more than likely turned in to cancer/ sarcoma. She is too old to try to have it removed. Has anyone else experienced this with their cat. He did drain a good bit of fluid off the area but now it looks as though it needs it again. I cant afford to take her to the vet everyweek to have it drained. Not sure what to do next.

I'm so sorry you're going through this.  Since you're a nurse yourself, would draining it be something you could do yourself if shown how to do it?  Have you considered getting a second opinion?

I was able to aspirate some fluid but it does have more of a bloody appearance. I spoke with another Vet/friend  and she says this type of cancer is very aggressive. it will spread quickly. I am just going to make her comfortable as possible. She is such a  great cat and friend. I hate to lose her. I will

keep her at home unless she gets actively worse which I will go back to vet to may be get some other form of pain meds. Thanks for listening
This was taken 3 years ago. Her name is Sugar