Did the vet leave the area open to continue draining? Once lanced, abscesses will continue to produce pus, it's important to continue to clean them and rinse them and let them heal from the inside out. Take a look at the opening to see if it has started closing over... I have found that if it has started to scab, hydrogen peroxide will soften it and make it easier to open the wound again. I usually flush abscesses with sterile saline in a syringe (the kind of contact lenses is good enough.) Gently put pressure around the opening to express any more pus and do this a couple of times a day. It may also be necessary to look into other antibiotics. Baytril is the go-to antibiotics that many vets prescribe for guinea pigs, but it generally isn't that effective against abscesses. Zithromax is supposed to be best, I have also had pigs treated with chloramphenicol for abscesses with good results.