First of all, if she's not used to eating something and you give her more than a little tiny bit, that's enough to upset her digestive system. something like fresh pineapple, if you give more than a little bit at a time (like a small chunk), because of the high water content, is likely to do that.
Just feed hay and pellets for a little while, and then, if you have to feed her pineapple again, VERY small pieces over a long span of time.
If ever the diahrrea keeps up for more than a day, you should take her to the vet immediately so that you can stop that. Rabbits seem to have a really difficult time maintaining their water levels and so diahrrea can actually dehydrate them to the point of death.