Take her to the doctor to be sure that it's only a sty (which is bad enough, I know!). When my daughter was about 1 1/2 or so, she had a problem come up called periorbital cellulitis (basically a skin infection in the eyelid) which is easily treated with a strong antibiotic, but can go on to quickly damage the actual eye if untreated. The cellulitis is actually common in little ones and can come up from them having a small scratch or owie near the eye or on the eyelid (a scratch that we might not even notice). The only symptom that my daughter had when she awakened the morning I noticed it was one eyelid was swollen and reddened, but she seemed to feel fine otherwise.
Not meaning to sound scary or say that your daughter has this, just saying "better safe than sorry" in this case - anything with the eyes is so important to get checked out!
Hope all is well and that your daughter's eye is all better soon!
S.