Mass in bladder on US

Question
I have a dwarf lop ear bunny I adopted.  She is about 6 years old and has been very healthy.  I only feed her Timothy Complete pellets and Timothy hay.  This past week, I noticed she wasn't eating quite as much of her pellets and then over the week end, she didn't eat anything or drink.  Her respiratory rate increased to about 80-110 and of course no energy.  I gave her water via syringe and she tolerated that.  Still making urine and very small pellets - good bowel sounds.  I took her to a vet and he performed an US of the bladder and informed me he saw a mass.  He is unsure as to whether it is a tumor, infection or stone. He said he could actually palpate the mass.  He gave me an antibiotic to try for 2 weeks and I am giving her Critical Care feedings and water by syringe.  He did mention surgery if the antibiotics don't work. I don't know of any vets here that specialize in bunnies and I want to know if we are on the right track.  Is it possible that this could be a bladder stone or infection?  I am willing to do whatever she needs if it will help her, but I don't want to put her through any unnecessary suffering. Are there any other tests that should be performed to see if this is a tumor, if it has metastasized elsewhere? These symptoms seem to have had such a sudden on-set.  I would very much appreciate your thoughts and recommendations.  She is so sweet and we love her.  I want to do what's right for her.

Answer
Dear Vicki,

You may be able to find a rabbit-savvy vet here:

http://www.rabbit.org/vets

and it would be worth a drive to get her to someone who can safely do a surgery.  

Stones are not all that uncommon.  If the mass is moving around, then it is likely a stone, not a tumor.  

Is she spayed?  If not, I wonder if this might be a uterine tumor that the vet misdiagnosed.  Uterine cancer rate is quite high in unspayed females, so this is a possibility, too.  If it is a uterine tumor, then spaying her will usually solve the problem.  These masses do not tend to metastasize very early in their formation.

A full-body radiograph might help the vet detect any metastatic masses, if they are large enough.  That might help you decide a course of action.  But if this is a stone, surgery is probably the best option to remove it.  And you'll want a vet who is experienced with rabbit surgeries and anesthesia for this type of procedure.

I hope this helps.

Dana