Cat urinating in guinea pig litter box!!

So I recently moved and my kids have guinea pigs with a home-made cage that is on the floor, it does not have a cover, and really cant be covered.  Lately my cat, who loves jumping in there and rolling around like a dog in the grass, has started peeing in their litter box...this is becoming a problem, since it is a very small litter box and once she pees in there, its full....not to mention its wasting their litter, making them pee in the cage itself and I just don't understand why she is doing it!  We have 3 cats and 3 guinea pigs...she's the only one who jumps in the cage. She never has had any problems with the litter box and the litter box is always clean.....always. I added a second litter box but she still goes in there.  I know I should have like a bunch of litter boxes for 3 cats...but I have one, don't have space for more, and I don't need more they have always been fine.  Until now and adding more litter boxes hasn't helped anyway.  She doesn't poop in the guinea pigs litter box but I'm getting nervous that that will be next! I don't really know what to do, this just started happening and I am not home every minute so I cant kick her out of there. Oh, and there is no way to keep her out of that room, there is no door.  So.... any thoughts???

I don't think there's anything you can do. Anything you do to try to make her not use their box could result in her not using ANY litterbox. So blocking her access would be the only way. Maybe you can build something to keep her out? I built a cover for my tortoise enclosure with cubes and zip ties.