Guapo's cancer presented that way. He was totally fine, just had a cough. They thought it was kennel cough. Antibiotics didn't clear it up. Shortly after that, we found out it was lung cancer and he had a massive tumor in his lung. We got 2 more months with him. It really felt like it came out of nowhere.
I can't imagine how you're feeling right now. Be kind to yourself.
"Why would you ruin perfectly good peanuts by adding candy corn? That's like saying hey, I have these awesome nachos, guess I better add some dryer lint." - Nonny
I'm so, so sorry! Please don't feel like you need to apologize. When my old cat died, it was similar. He had been at the vet a few months before and was doing well for a 16 year old cat. I got refills for his IBD meds and I thought he'd live for a few more years. A few months later, he got really lethargic and I took him to the vet the next morning. He had a huge mass in his abdomen. The vet said it was the biggest she'd seen in a cat. I also lost a dog to a tumor in her spleen. She went from seemingly healthy to vet ER in about 36 hours. Anecdotes and all I know, but it's so true. It can happen so, so fast. My heart breaks for you.