Well, even if there is no puncture in the skin, I can't rule-out that a abscess is not forming. However, with no penetration of the skin, an infection is less likely. So, two other considerations would include:
1. Hematoma (so a blood clot formed under the skin)
2. Seroma (like a cyst. It fills with fluid due to an inflammatory reaction).
Either way, a lump certainly doesn't belong there. Definitely, well worth a vet visit since we doin't know for sure if antibiotics should be started immediately or not.
If the vet is unsure, they may suggest a simple needle poke test to see what drains out of it to determine therapy.
Not a great idea to take a wait and see approach, since infection can't be ruled-out yet.
Hope that information helps