I dont want my bunny to suffer, how do I know if hes in pain?

Question
Our 6 year old bunny bled a lot today by his tail and we rushed him to the vet. The vet found a tumor but told us it's too close to his anus and can't operate. The vet said that we might have to euthanize him, and that we should watch him for changes in behaviour. We love our bunny very much, and don't want him to be in pain. I know rabbits are very good at hiding they're sick (we never noticed anything different until he started bleeding), so how do we know if he's in pain?
He's been eating normally, but his breathing seems agitated. We don't know if it's the stress of taking him to the vet today, or if he's in pain.  
What should we do? we want to do what's best for our bunny of course, but we don't want to euthanize him if it's not necessary. But we certainly don't want him to suffer, what should we watch for?
The vet didn't prescribe any drugs or anything... should we look for a second opinion?
We are all very sad.

Thank you.

Answer
Dear Patricia,

I hope you might be able to bring the bunny to a different vet for a second opinion.  This might be something more treatable than a tumor, and the bunny should certainly be getting pain medication, such as Metacam.

Please check the international listings here:

www.rabbit.org/vets

for a vet who might be better able to help.

Dana