R.B.
Hi J.,
I'm a pediatric Speech Language Pathologist. I think you have a great ear and are a keen observer! What I would recommend without seeing her is to not "call attention" to it, but overpronounce the /s/ sound with a big smile. You can even play a "snake game" one morning and point out how your teeth are together so you can make a big snake sound (or you could say my tongue is hiding, and my teeth are together)... but don't ask her to perform it... she might just want to try it out if you don't put the demand on her. Make it silly.
Just for reassurance, I will say that /s/ is one of those quirky sounds that can take all the way up to a later age to properly develop, but I never tell parents to just sit and wait if you have concerns. Try out some speech sound games without being overly forceful in having her try it... and just do some nice modeling of the sound and see if she catches on first. Otherwise, absolutely bring her in for an eval if you feel more comfortable. I personally would not pick up a 2 year old for a lisp - only because it's developmentally appropriate. At the same time, the earlier you can make her aware (without making her self conscious) the better. It's a tricky balance, so focus more on showing and modeling correct production rather than having her try to "fix" it or putting her on the spot. The good news is, she said it correctly before, so I'm sure she has it in her to say it correctly!
Is she a thumb sucker? There is no pacifier any longer right? Just food for thought - those both can cause changes in palate and tooth structure than can cause a lisp that is not correctable with traditional therapy.
Good luck to you!