Girls, when do you do an inward lip roll (hide lips in mouth)?

It happened today, three times, and has been happening a lot lately. I would assume it might be cause of nervousness and stress, but I do not do it myself so I would therefore get your expert opinion on it. Usually happens during or right after eyw contact/they glance at me. Seems somewhat weird to me considering how easy it is to spot, that's why I ask. Thanks :-)
Girls, when do you do an inward lip roll (hide lips in mouth)?
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