This is a male cat I found on the road; it had a lump that grew much bigger in just one day

From the photo, a swelling behind the ear that grew the next day has several causes. First feel whether it’s soft or hard.

If soft inside, it may hold blood or pus from a burst capillary or bacterial infection—an abscess needing a vet to incise, clean, and drain, then ongoing medication and an e-collar.

Skin infection can also cause inflammatory swelling; a skin scrape identifies the type. If the lump is hard, grows fast, and doesn’t move much, it may be a tumor—the vet will biopsy to tell benign from malignant.