My feeling is that its a new trick. Will has been through loads of things like this - including lip smacking, sticking his tongue out, raspberries, shouting etc etc...
It could also be a sign of gum discomfort/ teething - 3 months isn't all that early when it comes to teething - the teeth start pushing down through the gums right from the start in some LOs - Will was 8 weeks old when he had the first signs - although a tooth didnt actually appear until 8 months! You can get teething gel that is usable from birth - pop a tiny bit on a see if it makes a difference.
I am pretty sure but I doubt your LO is thirsty of she is taking all of her milk etc.. but I guess you could offer her a drink of water just in case.