Difficult one to guess at. They get lumps and bumps of all kinds, just as we do. Could be any number of things, including a cyst or an abscess. I'd just keep an eye on it at this stage. It may burst of its own accord, or when caught on or rubbed against something. If it gets much bigger, I'd call your vet, who will probably stick a needle in it and withdraw to see what it contains. There are things you could yourself, but unless you're very confident or experienced with pigs, I'd leave it to the vet.