Sounds a bit like migraines. They don't always hurt, and can manifest as blurred or reduced vision.
My granddaughter is 7yrs old and going through sudden blurred vision as well, with some headaches off and on. She has been to three Opthamologists with no answers, they haven't seen anything like it, they say she is NOT faking it, she can't see to read with or without glasses. She has had several tests. Have you had any luck or answers with your daughters vision?