What Causes One Eye To Be Smaller Than Other With Puffiness And Swelling?

I have an 8 year old daughter. I came home from work today and noticed that her left eye was smaller than her right eye, the dark circle underneath was puffy or swollen, and the inside of the eye was red. I thought she rubbed it but she said she didn't.. I took her to the pharmacy since her pediatrician's clinic was closed and the pharmacist suggested i buy refresh tears, eyedrops, she said it looks like something she caught from the outside. But i kind of put two and two together, i am thinking it could be a sinus infection. she had this about a year ago. to make a very long story short, what do you think it could be and what could i do..