K.M.
Where she picked up this new habit isn't as important as to why. Most children suck their thumbs and even their arms for comfort. Something has made her uncomfortable.
From what I've read, don't worry about it. Do NOT put bad tasting things on her thumb in hope she'll stop. That's just mean to her. She wants comfort, not her mommy putting nasty things on her fingers.
She'll stop in time. It's normal for some children to suck their thumbs into school age. It won't do her any harm, so why stop it? Let her have her comfort thing. It'd be like taking her favorite toy away.