Since you've eliminated the medical cause it has to be that something is up. Something is bothering her. Are the accidents getting her some attention from you that she's looking for? Have you ahd you husband been fighting? Have you been crazy busy at work, with a sick family member or on a project and haven't been able to spend as much time with her? Of course, there could be something scaring or intimidating her - a kid at school - a teacher or aide?
Some kids are hyper senstive so a slight change can cause it. If her teacher is not well or is dealing with something and is frowning more (becuase she's thinking of her mom's cancer for example) your DD may internalize the frown as "teacher is mad at me and doesn't like me".
Kids are funny - they can't put in to words the stuff that's bothering them. They can't figure it out themselves. We always jump to conclusions that it's something awful when it can be something that's happening to someone else. As you're driving around town on errands with her ask her if thre's anything that's making her sad or scared? Tell her about stuff that's made you sad and how you thoguht it was one thing but it ended up not being a big deal (I thought grandma was mad at me one day becuase she had the flu and was getting really sick but didn't realize it and she looked like she was mad - but she was really just not feeling good...)
Chances are the accidents are bothering your DD too - so ask her what she thinks. Make sure it's realy nonchalant - you don't want her to think you'd mad or disappointed with her.
Good luck mama - this is the stuff of life that can make us nuts.