Your rabbit's lump is an abcess. An abcess is a capsule of thickened inflamed tissue containing pus caused by a bacterial infection. Most facial abscesses are the result of dental disease.
Since the pus in abcesses is too thick to drain, most abcesses need to be removed surgically if they are accessable. If they aren't, other treatment methods include injecting the wall of the abscess with antibiotics at periodic intervals, or packing antibiotic beads into the cleaned abscess cavity to kill bacteria.
You should take your rabbit to a vet who treats exotic animals to have the abcess evaluated and determine the best way to treat it. If you don't already see a vet for exotics, you can find one in your area by going to this website:
http://www.veterinarypartner.com/Content.plx?P=A&A=503&S=5&SourceID=43
or look in the yellow pages under vets, exotic.
See:
http://www.veterinarypartner.com/Content.plx?P=A&A=503&S=5&SourceID=43
