My dog sometimes wakes up at night and walks around the house?

We had our dog for a few months now. She's almost 4 years old. Up to now she usually slept well, beside us on her bed, and sometimes changing position to be on the floor where it's colder. Nothing out of the ordinary.

But now she's acting weird. Sometimes at night she's waking up and starts walking around the bedroom back and forth, like she's searching for a place to sleep, getting out of the room, into the other room, sleep for a few minutes, and goes back into the bedroom. Sometimes just for a few seconds and going back out, then in again.

I'm so confused as to why she's doing this.

Any idea?