BF thinks I cheated

Boyfriend of 3 months went to visit his family for a couple of days for Christmas. And when he came back he made a face and asked me how I got my cold sore on my lip. I’m prone to get them during the cold season. When I explained he said you usually get them from kissing. He got moody with an attitude. I didn’t do anything except work and be with family. This is really getting to me that he’s assuming and not trusting me. What can I do?