Dog crying at the bottom of the stairs

This might be a weird one. My little girl 3 years old sleeps with me 90% of the time, sometimes she likes to be independent and sleep on the sofa. However, when she does sleep downstairs I can hear her crying/whimpering at the bottom of the stairs at about 4am which breaks me immediately out of sleep. I go downstairs where it’s still dark thinking she needs to go outside for her business but as soon as I step down she runs upstairs straight into my bed! What is this behaviour?!!