Waking at night

My 5-m-old girl was just starting to improve her sleeping at night, but she is now worse than ever! She wakes up nearly every 2 hours. Sometimes she goes back to sleep on her own, sometimes she doesn’t. She is on three meals a day, so I don’t think she’s hungry, and there are no obvious signs of teething.She is not ill, we had a check up a few days ago and she is perfectly healthy. What is bothering her?