L.,
My daughter is the same age (born on Oct. 5, 2006)! I know it seems a little drastic, but I would guess that your doctor wants your daughter in the ER to make sure she's not dehydrated. Little ones can become dehydrated very quickly, and it can be really dangerous. A friend of mine is really close to a family who's 2-year-old died last Christmas because he had a stomach-virus and became dehydrated. That was an extreme case, but I'm just saying it can happen.
My daughter had a double ear infection a couple months ago and wouldn't drink anything because it hurt to suck/swallow. She was the same as your's... she would eat only a little, but not drink anything. When we put her to bed one night, she had a 102.6 fever and no wet diaper. When she woke up in the morning, her diaper was still not wet. She stayed with my mom while my husband and I went to work, and at noon, my mom called to say Emma still had not had a wet diaper. I called the doctor and she said we needed to get her in right away because she could be dangerously dehydrated. I called my mom to tell her to take her in, and she had just changed a wet diaper, so we ended up being ok.
I guess what I'm trying to say is, if your little one isn't taking fluids, she could become dehydrated, which can be very serious. Since your doctor said to take her in, I would, just to be safe. You'd hate to regret it if something happened and you didn't take her in!
Good luck! I hope your little one gets to feeling better!
-S.