dd just started thumb-sucking at 21 months

[JiminyCricket]

and I think its because she's teething badly, suddenly its night and day. She's never sucked her thumb before and never bothered with dummies. Would you (can you) try to do anything to prevent it becoming a fully-formed habit, wait and see if it eases off with the teething, or wait until she's older and tackle it then if necessary? I'm more suprised than bothered, btw, just because she never has before. Any thought or experiences gratefully received.