This could be a cyst, ingrown hair, infection or it could even be a warble (parasites that burrows under the skin).
For tonight, I would wash it well with a betadine or peroxide wash. Then dry it completely and you can apply some neosporin ointment.
If you don't see it start to resolve, you will want to see your vet to find out what's going on.
Let me know if you still have questions.