Well, then the lump needs to be examined and pinpointed as to what it is. Cephalexin isn't going to penetrate a pus pocket like that; you would need an anaerobic drug like Antirobe for that, or a combination of clavamox and metronidozole. But I still would investigate what is the exact nature of the bump first, before slinging (the wrong kind of) antibiotics in the dark.

If it is an abscess, then you might be dealing with a bad tooth or infected salivary gland, for which (again) Antirobe (clindamycin) is the preferred drug.

But it always pays to know, not guess.