She could have already given birth and you didn't notice. In my experience they usually give birth in early morning, and the fry are very tiny and get eaten by other fish very quickly.
This could be a possible explanation of her return to normal behavior. Also, they don't always drop all the fry at once. Sometimes it's sporadic with 5 fry one day, 10 a week later, 2 a week later, etc. in my experience it's not always perfectly regular like you read about (once a month 30-50 fry).