Does the lump feel solid or sort of squishy?
Years ago
Cally had a small local swelling on his hind leg that I couldn't figure out why, it also looked like an infection but drained without "normal" solid white pus - it was mostly "watery" blood w/tiny pockets of liquidy pus. Never did find out what it was from, but figured he must've scraped it on a fence & got a splinter or something to start it all.
Speedy healing Marv