My 5 1/2 year old has just STARTED to suck her thumb/ fingers??

She’s never done this her whole life and all of a sudden I always catch her sucking on her fingers or full on sucking her thumb like a baby and quite honestly ( I feel bad for saying this) I have no idea why, it ENRAGES me!!!

Like I don’t know why but it triggers the hell out of me to see her just going ham on her thumb lol. I tell her to stop and she’ll stop but why is she just now starting this habit? Is this like a cause of concern?

I asked why or if she’s seen it anywhere and she just said “don’t ask me that” “stop” “no” lmao