I'm so sorry!
Did she have any other symptoms?
Sneezing? Labored breathing? Loss of appetite? Diarrhea? Lumps on/under her skin? Anything??
Usually, rapid weight loss is caused by cancer or renal (kidney) failure. It could also be age-related...how old was she?
Was she drinking? I have had mine go off feed due to waterer issues. Some just hate the waterbottle. Also there are weeds that are toxic... from what I gather you don't feed pellets?I just thought I would add that she was more like wasting rather than just being skinny. I was feeding them mixed weeds and hay, and then mostly hay because they preferred the weeds and started trying to get out to get more and wouldn't eat the hay. So I only gave them weeds twice a week and then they started eating the hay. Anyway sorry for rambling the point is I was always worried about them getting skinny on weeds and hay especially since once it got hot I started giving them enough for two days and not going down the hill everyday. 5 days before she got so skinny my sister was down with me and she was petting everyone and feeling their weight for me. She pet BB and felt her and said "she's a little thin but definitely not that bad" and I was like okay so I should feed them more hay. And I did and then all this happened.
I honestly don't know what's going on with your rabbits! I'm sorry, it's just such a mystery!I think she was drinking water. They have two bowls of water and a old chicken water but I scrubbed that thing like crazy and they drank out of them for months. I didn't know they wouldn't drink if it got warm. That's interesting. The water level was going down but I don't know if that was just the other two.
I didn't feed pellets but I do now. I was trying to get BBs weight back up and I am trying to get Springs weight up. She's not bad just a little thin. Hopefully she's not sick or if she is hoping she recovers.
I had them on just weeds and hay for a while and they were all healthy. Until about two weeks ago the baby rabbits died and now BB. (Keep in mind BB , Flinn, and Spring are/were in the same pen and ate the same thing. Flinn isn't skinny at all and Spring is thin. While BB got emaciated within 5 days.) I have a litter of six meat rabbits that got fat off of hay and weeds.They where eating weeds since they could and had been up until about 10 weeks because we got to busy to pick enough weeds for so many rabbits. And BBs litterof 10 was fed just weeds up until they were four weeks old and I knew that others wouldn't pick weeds everyday for the rabbits so I started to transition them to hay and pellets.
They would eat most of the weeds. Some they wouldn't but then I wouldn't give it to them anymore. And some they would eat the leaves but not the stem.
On a side note have you ever had someone just steel rabbits. Someone stole three of our six meat rabbits a couple days ago.