Why is my 6 month old smacking her lips?

Je

My baby has been smacking her lips for the last few days. She's been doing it a lot, all day. Could it be teething? She got her bottom teeth about 3 weeks ago. She didn't smack her lips like that before. 🤔

Edit: she does it even when she's well fed. She'll do it right when she wakes up too, while she's still sleepy.