It's very likely an ear infection.
This is exactly what happened at our house on Saturday night. It can happen that quickly. My son went to sleep fine and woke up saying his ear hurt and wanted a bandaid. We had a doctor come to the house and immediately told us it was an ear infection. The doctor gave us amoxicillin and said it was important to give him Motrin for the pain. He started feeling better once the Motrin kicked in and was back to himself by Monday.
Warm olive oil can also help relieve ear pain. If oyu can get your little one to sit still long enough to get it in her ear!