What kind of infections do indoor cats get?

I’m a bit worried and confused tbh. My cat had a high fever (42°C) and was given antibiotics for an infection. She’s been to the vet twice over the last few days, first time with my mum and this time around, I went myself, just for a check up as cats don’t talk lol, and I wanted some answers too. I asked the vet what could have cause such a high fever and he said that it’s very unusual for an indoor cat to have such a high fever. And this makes me feel somewhat guilty or idk how to feel, like what could have gone wrong. I feed my cat well, her litter is clean, and she’s vaccinated. I’ve not felt great over the last few days, between just seeing my usual goofy cat just being quiet and the guilt I feel. I have no idea what could have gone wrong. Has anyone ever experienced this?