Sudden Weight Loss

I haven't been on in quite sometime; since then, my rabbit was unfortunatley given to another home due to moving out to college; ANYWAYS. My roomates I rent from have 6 rabbits.
One of there females has had sudden weight loss; she has huge dewlaps but we can feel her hip bones, ribs, even her legs feel smaller.
Recently her cagemate was reintroduced (her mother (now spayed))-- little scuttle in the beginning but now they're fine; what are the possibilites? What do we do?
Of course we're thinking of going to the vet soon, but for the meantime, what could have caused this? It seems to have happened quickly. A few months ago all the rabbits caught Coccidiosis from the 2 adopted rabbits from the shelter. They were all given antibiotics to rid of the bacteria. They all got better and everyone healed well.
She's eating no problem and pooping and peeing normally, cancer? Cagemate eating everything?
Thanks!! Any ideas will help!

First off would be to check the rabbit is eating her food well and is getting her portion of pellets, hay, etc. If the rabbit is eating her share and there are no recent dietary changes that could explain the weight loss, there are many health problems that can cause sudden weight loss in rabbits. Such as dental problems, infection, cancer, organ failure, and parasites, including e. cuniculi as well as coccidia. Due to coccidiosis being a recent illness occurring, that may be a good place to start and rule a reoccurrence of that out with a fecal test. But I would definitely suggest your roommate get the rabbit to the vet right away to get checked, as the cause of the weight loss could be potentially very serious.
http://www.rabbitnetwork.org/articles/weightloss.shtml

Alright great, thank you. She's off to the vet tomorrow morning She's 6 yrs old. and has been eating the same amount of food. Her mom had cancer and was spayed and she has no longer had any signs. But I know the possibilities are always high. Thanks!