J.,
She is old enough for you to have a good talk with her about hygeine. Teach her what she needs to know. It could stem from a lot of things loss of her father, your new relationship, lonliness or laziness. Sit her down and talk first. Make her understand that if she doesn't clean herslef, she is letting herself in for a whole host of medical problems. Infections, rashes, even kidney disease. I have a granddaughter who didn't want to clean herself. Before we knew it, she had a severe kidney infection and now she has to be closely watched so she does clean herself properly. She is only 6! She also cannot take baths she has to take showers so she doesn't sit in any type of dirty water.
Also, keep in mind that any time now or in a few years, she will be starting her period. If she doesn't learn good hygeine and keep up with it now, what will happen when that starts? You are the mother, take charge, you make the rules not your 10 year old. Good luck.
E.