Shih Tzu (4yo) has been gagging/throwing up once per day. Any recommendation?

Every day since Friday, my Shih Tzu will gag and throw up a little bit of kibble. It’s right after her afternoon nap and when she comes down the steps that are attached to my bed.

She’ll continue to gag for like 2-3 minutes and then stop. She’s fine for the rest of the day. She runs around and plays, eats, drinks, etc.

Does anyone have any ideas on why it’s happening? I’ve phoned the vet but they are unfortunately closed until tomorrow.