my 4month old kitten has a red eye

Hello I’m a first time cat parent I first found a kitten (Lily, now 5 months) and recently we adopted another one ( Ash, 4months) they get along fine but they playfight a lot and Lily always has her claws out even when she’s playing with us.Well this morning we woke up and noticed that Ash, one of her eyes is red and she can’t fully open it. we don’t know what it could be since I had woken up earlier for water and her eye seemed fine we think maybe the cats playing rough or something. We are taking her to the vet today at 4 but I’m just really worried, she doesn’t seem in pain or anything they were playing just fine and she was purring with us but we still separated them because I didn’t want them play fighting and making it worse. I’m just really worried because her eye seems really irritated and she can’t open that eye fully she keeps running into things. if anyone knows any reasons as to why her eye is like this I’d really appreciate it (just for my own peace of mind nd to see what i can do to help in the meantime) and i apologize for the long rant