Hi! foot swelling in a diffuse pattern especially around ankles is a sign of edema or water retention but water retention should not present as a localised lump...! However, this lump is easily palpable therefore easy to diagnose clinically and determine the cause behind it; the various causes could be gouty tophus, synovitis, cysts, abscess etc. etc... Therefore you need to see your physician for a clinical examination and possibly to undergo a needle biopsy for confirmation and management! Take care; regards,