Your piggy is kinda young to have developed an abscess, it could be a fatty lump, but then again nothing is impossible. If there is no sign of distress or lethargy there's a chance of it not being something bad...Just in case, keep an eye on the lump and watch out for any bright pink tip - like those on developing pimples/ anything that resembles a freshly scratched mosquito bite -. If any part of the lump resembles one of the above, then it's an abscess.
It happened to my first piggy: he suddenly developed a medium size growth - 50 Euro cents diameter - which moved under the skin, it was a hard mass and it caused no distress in the beginning. After performing an ultrasound the vet couldn't decide between fatty lump or tumor, gave him antibiotics and told me to monitor Rontzi. Nothing happened with antibiotics and about a week after the treatment the lump developed a pinkish tip just like a huge zit; in the meanwhile my piggy became lethargic and wouldn't eat munch, couldn't stand my touching him etc. The vet made an incision, cleaned the wound, applied local antibiotics for 5 days and Rontzi was ok.
Hope yours will also be alright. Best of luck.