I got my male cat neutered about 3 or 4 weeks ago now there is an open sore nickel-sized by penis area?

We just presently moved to a new apartment and after 3 days of being scared and stressed he settled in nicely. Now he is out and about and purring. Today I noticed this nickel-sized open sore right in the area between back legs. It looks pink and is not pussy or oozing. Any ideas? Does it have anything to do with him being neutered? That was 4 weeks ago, why would there be an open sore now?