4yr.old is peeing

My 4yr.old all of the sudden is having accident everyday. It's been a week this is happeing where she runs to the bathroom and doesn't make it. She sits on the toilet crying because she knows she has done bad and feels bad. I have talked to her asked her what is wrong ? she responds she doesn't know. I asked her if she is holding her pee, she said she is but she doesn't know why. I don't know what to do. Its not like her to have accidents at all. I am surprised by this and concern. Its to the point where she is changing her own underwear when she has an accident and/or she is lieing about going to the bathroom.  I need advice/help.