Toddler experiementing with making up stories. Should I worry? How do you know what to believe?

Okay, I came from a family with people who were so good to me that I never was abused or anything and neither was my sister. My uncle that I am going to mention in this story is the same uncle that was like a dad to us, that we spent the night with (him and my aunt) sleeping between them, and he treated us like his own kids. Okay, lately my son has been experiementing with making up little stories. Like, blaming stuff on the dog and telling me silly things that he knows isn't true just to get a reaction out of me. Also, lately, he has had an itchy butt. Just being what I thought was a protective mom, I asked him had anybody ever touched his bottom. Well, he said yeah. I said who. He said you. I said, no Avery I just change you (cuz that's all I do and bathe him) and so I said who and I started naming people (probably a mistake) he said my uncle. Okay, this freaked me out a little and I said what did he do and he said put a stick in my butt. I said put a stick in your butt???? and he said no, he put his finger up his butt. Okay, keep in mind my uncle isn't a perv and my child also told us today after peeing on himself that my grandma peed on him. lol (He is potty training). So, when he said my uncle stuck his finger up his butt it still worried me because I love my child more than anything and I wouldn't want to seem him hurt, but it just doesn't seem to add up. He is rarely ever with my uncle more than five minutes alone because we have a large family at my grandma's all the time and everybody when I told them, laughed it off saying he was making it up that my uncle isn't like that- which I would agree. Later, when I asked my son he said no it wasn't true. Then I asked him again and he said it was. Later he might say no. He is three. There are no signs of abuse and he loves my uncle to death. Should I just think of this as another one of his crazy stories? He has no signs of abuse what so ever, physically or emotionally. I just don't like that he said that. He probably wouldn't have said it, had I not asked him and named names. What do you guys think?