I'm so sorry to hear about Sydney. You must be very worried. I haven't heard of a lump forming so quickly but Shepgirl's description sounds very similar to Sydney's.
My cat Tiger has been battling Squamous Cell Carcinoma for just over a year now. I definitely understand your frustration in not knowing what is going on with Sydney, despite the 700.00 you've already spent. The biopsy will make everything much clearer though and hopefully you'll have the results soon. Tiger didn't have a biopsy done until 8 weeks after he started having respiratory issues and 5 different vets. They thought he had everything under the sun until I went to a vet who had a rhinoscopy (camera small enough to go into the nose) and he performed the biopsy. I'm just so glad that you noticed the lump right away and got her to the vet.
If you do find out it's cancer, you may be interested in a product that I've had my cat Tiger on for a few months. I wish I had learned of it sooner even though I don't know if it's helping but it seems to have helped quite a few other animals battling cancer. It's called Transfer Factor. (If you click on this link it will take you to the website) It's a mega immune booster supplement. I learned about it from Catty1 who posted a link for me to check out. I ended up reading some testimonials from some people who had used it on their pets with cancer. One woman had a cat with S.C. Carcinoma and I think she couldn't afford the surgery. Her cat was in terrible shape and she took him to the vet to be euthanized but at the last minute she felt it wasn't the right decision. She went home and found out about Transfer Factor. She gave her cat 4-5 capsules (of the human capsules, not the pet capsules) a day for 2 months and then tapered it off to 3 caps a week. I emailed her to ask her how long her cat had lived after that and she told me that her cat was doing fine and was still with her two years later. I can't tell you if it works or not from personal experience because he's only been on it for a few months and he's had a few setbacks with congestion and infection and I had to stop giving it to him but I've started him on it again today.
Before I put Tiger on Transfer Factor, he had had 3 surgeries to remove parts of the nasal tumour. Each time, it really helped him to breathe but it did grow back faster each time and I think Craftlady might be right about it growing back larger etc.. It certainly seems to be the case with Tiger. But, if it could provide Sydney with immediate relief, it might be the way to go.
And, it may not be cancer at all which is definitely what I'm praying for!!! I hope that it is some kind of infection that didn't respond to the first round of antibiotics but will respond to a different kind of antibiotic. That would be such a blessing.
Please keep us posted on your beautiful tabby girl.