Why is she doing this???

It has just started, yesterday I noticed it and she's doing it today too.

I know it can mean tired or teething, well she was doing it right after I got her up after a sleep today with no other tired signs, she kept doing it on and off until I put her back to bed just now. Can't be a tired sign then?? Or can it??

As for teeth, well I've had a good feel and there's nothing sharp or anything in her mouth.

What else could it be? She's 4 months old.