Yes, there is one near the Crossroads shopping center at the edge of the Downtown Disney hotels. If you are coming from DD and get to that intersection, you make a left and it is not even a mile up the road. I think it was called Florida MedHealth or something like that.
We were pretty sure our son had an ear infection so we called the front desk to ask them where to go. They said the doctor could come to our room so we just did that. He was from that walk-in clinic.