What could this circular lump be?

It’s behind his left arm pit it just appeared outta nowhere. When I touch it he doesn’t seem in discomfort, however teddy is a very strong quiet doggo he’s never made a sound or ever showed he was in pain even when he was in pain. Teddy has an appointment next week. However I keep over thinking it. My first doggo before ted have a lump like this and it ended up being cancer and she died 2 weeks later and I only had her for a year at that point, I was devastated. Teddy is my everything. I’ve had him for 4 years and I couldn’t imagine life without him at the moment. I keep having dreams about losing him and how I’m going to react. I just keep thinking about how the appointment is going to go.