Can you give some advise on how to pursue with my son's unusual behavior?

Hi doctor,

My seven year old son has displayed a very disturbing sexual behavior. As far as I know, the incident was isolated. I am unsure if this is a usual behavior for his age group or not. I thought he was playing video games when I grounded him from them, because he was up in the playroom very quiet. So, I snuck up on him, expecting to catch him playing games. When I looked around the corner, I noticed my son petting our pet cat, with his pelvic area against the cat. I was not sure what I was seeing and I was confused at first. But within a couple of seconds, he started to pull his pants down. He clearly did not know I was right there. He was erect and I am unsure what he was planning on doing. I just shouted and interrupted whatever he was planning. He got scared, was clearly embarrassed and ran to his room immediately. When I tried to talk to him about it, he would not tell me what he was doing. Instead, he just cried and said he did not want to tell me. The only thing I managed to get him to tell me was that he had a funny feeling down there. I am worried that this is a sign of something more extreme, maybe he has been abused. This seems like extremely unusual behavior. He does not have any other signs or symptoms of abuse and this is the only time something like this has happened. I do not know what to do. I told him that he has to keep his body parts to himself. If he has a funny feeling again, then he should never touch anybody or anything with it. Can you give some advice on where to go to get him help? Should I even pursue further help? Or should I just keep an eye on him and continue to talk to him about keeping our body parts? I do not know what to do, as this is unusual. I am embarrassed to ask anybody about this. I feel as though this is something that should not be overlooked. This does not seem like a typical situation. Please help.