I have no experience of treating lumps with medication, but I know that abscesses are often treated initially using antibiotics such as Baytril (Enrofloxacin) and Zithromax (Azithromycin). Azithromycin is definitely the most effective. Neither of my vets suggested meds for my pig's lump. Which meds did your piggy have? Some members have had pigs on Loxicom (anti inflammatory) for conditions such as arthritis for months or even a year or two. Other meds may be okay for longer than a few weeks too, if they are doing their job, I don't know for sure though.
My pig's lump was under a nipple too. It was just a single lump, and I think it was fairly hard. The second vet said that the fact that the stuff inside was thick and difficult to extract with a needle, suggested it was a tumour, but as I said previously, it turned out to be something else.