If it's not Pink Eye or a Stye you may want to condsider the following information.
If she has any type of allergy to pollens there is a chance she could have a small piece of pollen inside of her eye. It has been windy here the last two days. My son recently was riding his 4-wheeler and came back with the entire left side of his face swollen and his eye was completely shut. After several hours of trying to figure out what was wrong and taking Benadryl since it itched and was swollen we used warm compresses on his face and occassionally he would use and eye wash. Then one time he wiped toward the nose and a large piece of pine pollen came out and as soon as it came out the itching stopped and within minutes the swelling started to go down. So thinking of everything from changes in soap, laundry detergent,bug bites, pollens or a food allergy.
Good luck I hope you figure it out. Having dealth with 3 boys with allergies, I know how frustrating it can be.
L.