It may be where the umbilical cord was? Keep an eye on it and make sure it doesn't get worse. The kitten may need antibiotics. I would swab it with a wound cleaner for cats (at most farm supply stores like Tractor Supply) and observe it. It may be a hernia, though those are usually much bigger. If there is no colored drainage, the kitten is eating and drinking, observe it. If the kitten stops eating or fails to gain weight, bring her in.
Agree with di and bob
. It may just be the umbilical area healing, or may be a hernia. Time will tell. If it’s a hernia, a vet can fix it easily when the kitten is a few weeks older.
Right. Minor hernia without problems, are usually fixed when they are neutered / spayed, so no extra vet visit is necessary, just "a few dollar more" for the self surgery.