I agree with Suzi. With a sudden change in behavior like that, especially where she's not drinking the usual amount, I'd get her checked out by the vet. Small dogs can get dehydrated very quickly, and that can have severe consequences if not treated.
It definitely sounds like she's got a little tummy trouble going on. It sure wouldn't hurt to do what Suzi said and put her on chicken and rice for a few days, with as much water mixed in as you can, and still have her like it. (like thick chicken with rice soup)
Another thing that you can use, but I'd want to have her checked out by the vet first, is 1/2 Pepcid AC twice a day. We've used this several times with Kodi (on the vet's advice) when he's had tummy troubles.
I think, because he trials a lot, he comes into contact with a lot more germs than the average pet dog, even though I never let him come in direct contact with the other dogs, sniff other dog's droppings or drink any water but his own. Still there have been several instances over the past 4 years when he has had mild tummy problems after shows. A few days of Pepcid and a bland (chicken and rice) diet usually set him right. But I've done this originally under vet supervision, and later with her knowledge and agreement that I was handling it correctly.
While a lot of dogs DO start to settle down more as they approach 18 months, it doesn't sound right for it to start so suddenly.
Kodi sez, "Feel better soon, little sis!"