When you say a lump - is it one under the skin or more like a sore on the top of the skin.
One of my girls suffers every so often with 'sores' under her chin - the last lot she had had one that grew quite big within a couple of days - she gets treated with antibiotic tablets which clears it up quite quickly. Haven't found any particular cause for them appearing but do keep a close eye on her as they can come on very quickly.
Have now resorted to using creams as she runs and hides when she hears the rattle of the bottle