Skin lesions on my cat

Question
Hi, Dr. Gotthelf.
We have a spayed female cat, about 9-10 years old.  About a year and a half ago she started having occasional "holes" in her skin, always on her right side.  The vet thought they might be puncture wounds from other cats, as she was an outdoor cat.  No big deal; they'd heal up.
They began to get more frequent and we determined they were not from puncture wounds as we are now keeping her inside; we moved out of state and she is always an indoor cat now.  However, the lesions are no longer round puncture shaped, they are somewhat elongated and they weep or drain.  We had her put on a strong antibiotic, which helped, but the vet also noticed she has what may be a tumor of some sort on her left side; she looks lopsided.  I don't know if the two are connected.  We did not have a biopsy due to expense.  
Now the lesions have spread to around her backbone and to her left hindquarter, and the skin on the left side weeps and develops a crusty covering, which is different from the right side.  Today I noticed a foul smelling pus on the left side.  I can't find anything online that describes the condition she might have.  Her appetite is good, she seems normal for an older cat, her bathroom habits are pretty normal.  We cannot afford expensive surgeries or treatment and I think she's beyond the help of antibiotics and she's not really healing up.  I'm worried our only alternative is to have her euthanised, but I hate to do that if she's otherwise okay.  She doesn't seem to have any parasite problems or any food issues; she's rarely sick.  I'm wondering if you can help.

Thanks.

Answer
Unfortunately, the only way to know what this is is to have a biopsy or a culture done of these lesions.  If there is a tumor, an infection, a foreign body or a parasite, the pathology can tell you that. Without an examination I cannot tell you what this might be.