I really don't think there is anything wrong....some kids just have a hard time holding their "business" in. I went through this with my daughter and I also thought there was something wrong and took her to the ped. She told me that their little bladders are so small they can't tell they have to "GO" until that second. I had to go back to putting Pull Ups on my daughter at the age of 3 (She is 4.5 now) becasue she just would wet or poo herself all the time. But as she got older she is fine now. She still wears Pull Ups to bed but we are trying to break that soon. Try not to make a big deal about it and it will come in time.
As always if you are considered call your ped. that's what they are there for