Why didn't the vet try aspirating the lump before trying meds? You seem very confidant that it's just like the last time(abcess).I would have the vet try aspirating the lump,probably needing to insert a shunt so you can drain it a couple of times a day with a solution.I would also resume hand feeding if your piggie is losing weight.good luck. Try searching on
http://www.guinealynx.info/forums/index.php They have fantastic information from several vet techs and rescue people who have seen it all and then some.