If she had diahreeah, it is one of two things: Something she ate, especially lettuce could possibly do this. The reason why lettuce more than other greens is because it does not have alot of substance, but fills them up quickly on water.
More likely she had something else, a disease, especially if she was already thin. It could have been coccidiosis (sp?), or clostridium (I had this once with my bunnies).
Coccidiosis is easy to treat, thankfully, but the clostridium isn't, it's untreatable. The clostridium isn't as common, although it is a relatively unknown disease (the only reason we found out we had it from MULTIPLE rabbits we had gotten from a slew of rabbit breeders is because we sent it to a lab).
It could also simply be from greens, which I am not sure if you give or not, but are one of the dangers of them. Usually there aren't problems, but it can be, and when we sold rabbits, when our herd was a closed herd and clean, we sold pets to petshops. We didn't have snuffles, didn't have ANY diahreeah in ours. We would send them to the petshops, and they would give them greens and carrots, and they would die. So I don't have alot of experience, but I know generally speaking it isn't worth the risk. The pelleted food has a perfect vitamin and food content, and to supplement with grass or timothy hay just increases the fiber, which is fine, but anything other than that can be a problem. That being said, we did give them carrot greens here and there, but it was a tiny amount, like a couple of "strands". So anything in temperence should be okay, but stay away from the "watery" greens, those cause the most problems.

Parsley and carrot greens are pretty good, as are carrots, but all in small amounts they can eat within a couple of minutes.
Just going off the top of my head! Without seeing bunny it is hard to say exactly what happened, jut putting out all the possibilities here! I hope that helps!
Natalie