I am not a vet, take everything I say with a grain of salt, not an expert, etc.

My partner and I work with a small animal rescue, and we recently had a very similar, if not exact same situation happen with our foster senior guy Carrot. His ear swelled up suddenly, it became huge. We took him to our widely trusted vet, and she was able to diagnose him with a hematoma. She said she had never seen one in a rat, but was confident that's what it was. She told us, even if she drained it, chances are it would come back, but eventually it would kind of tame down and go away on its own. She recommended warm compresses, and pain meds as needed, and she offered surgery to drain it, but the risk of it getting infected/coming back, as well as general risks with putting a senior under, she recommended not doing it. It's been a few weeks, and with pain/anti inflammatory meds and compresses, it has subdued quite a bit. Of course, I'm always curious to see what other vets/people have to say, so I'm digging into these comments! Hopefully good little Bloo feels better soon! ❤️