8 year old daughter is wetting herself

This is a bit awkward, my 8 year old Kate, wets herself. I got suspicious last summer because she kept telling me she didn't want to go when she obviously did. After school one day I went up to her room she was changing and I could see her knickers were wet, she told me she had done it on the way home because she couldn't wait, so I let her off, but the following week I caught her hiding behind the garage stooping down, I could see what she was doing, she jumped up, and when I asked her what she had done she shrugged her shoulders and said, I was weeing my knickers, I asked why and she said I wanted to. I think shes playing some kind of game, because if she comes in and I tell her to make sure she uses the toilet, she says I don't need to ive done it in my pants, its as though she likes it. where do I go from here ?.