How old is your bunny? Sometimes as bunnies age they lose weight.
You should take her to a Rabbit veterinarian ASAP. Does she have mushy poo's? I am thinking she may have an intestional parasite which is easily treatable with oral anitbiotic. DO NOT take her to a vet that has not had the training for rabbits, The medicine a vet would give a cat or a dog to treat this condition will kill a rabbit.
She could also have fur mites, does she scratch a lot? This is also easily treatable. Brush her fur well, you do not want her to swallow any of that fur as she grooms herself, hairballs in rabbits can be dangerous if left untreated.
Only a good rabbit vet will be able to say for sure what is going on, please don't delay in getting her seen.

Good Luck.