Cat's Acting Weird - Dilated Eyes?

I got my sister a cat for Christmas and we have had her for several months now. She does not look like a normal cat, she has a kind of squished nose with a small face and huge eyes. When we got her, she was not very healthy. She sneezed, coughed, and wheezed all the time, but she is now a healthy cat. She is the same age as my cat, but she is way smaller. She eats often, and has a healthy diet, but today she has been acting really weird. Her eyes are extremely dilated, so much that her iris is barely visible. She has been crouching low to the ground and sniffing everything, and when she hears a noise, she jumped really high. We can't figure out what's wrong with her. She acts super scared, like she's hallucinating but I don't think she ate anything bad. She is an inside cat, and she has only acted like this once, and in that case she hid behind a cupboard for about six hours. I have put her in her kennel to keep her from hurting herself, but every time she touches the walls of the kennel, she freaks out. No one in my house has any types of drugs that she could have ingested. Does anyone have any ideas of why she's acting like this?