How is Tilly today? Hmmm...We had a similar situation with Izzy over the holidays. Izzy does not eat much and is sort of picky. I tried a new food sample that I got over the holidays (EVO small bites) and boy did she get the runs. Then I looked at the package and saw that it was extremely high in protein - like 42% and that's very high for Yorkies..I think they should be about 23-25% protein - it's hard on their liver. Then I decided to try Life's Abundance (from Linda at FuFuFashioin) and then I mixed it up with her Back to Basics and I haven't had a problem since. Izzy also sleeps a lot and I too think it's partly due to the cold weather here. But I noticed that since I've been mixing the Life's Abundance with her Back to Basics she has more energy. I'll be weaning her off the Back to Basics once I use it all up and then switchher to all Life's Abundance. I don't know if it's the LA but you never know.
Hope our little Tilly is feeling better today....if she isn't then I would call the vet just to make sure.