Hello Fane81 and Tabitha!
Once again, thanks a lot for your replies. You guys gave me lots of hope. For some reason, last week, I thought my bunny was dying for sure, but after reading your replies, I realised that my bunny is still very alive!!!
So here's what happened at the vet: she confirmed what you said Tabitha, saying that she didn't mean that Thumper would certainly get cancer, only that her chances of developing it were increased since she had not been spayed. As for the bleeding, she thinks there is a good chance that it is not blood, but merely pigments in her urine. She says that as bunnies get older, their urine get more and more pigmented and that's perfectly normal. She wants to do a urine analysis to check if that's the case. If not, the analysis can tell her if she has a bladder infection, which would also cause bloody urine. If it's an infection, medication should fix everything. If it turns out not to be either pigments or a bladder infection, she says that x-rays will tell us if it's stones ...hmmm... I think she said in her bladder or, last possible explanation, uterine cancer.
I asked her about the possibility of Thumper bleeding because she was in heat and she answered that this is a widely spread misconception. She said that, if your friend's bunnies were not sick, they probably had pigments in their urine because, according to her, bunnies do not bleed when in heat.
So that's that. I still don't know shat's going on with Thumper, but at least I'm reassured that there are other explanations than cancer. Now, if I could just get Thumper to give me a urine sample, that would be great. Any advice? I thought it would be a simple task: after leaving Thumper in her cage without bedding, she left me a little puddle of urine which I could take a sample from, but when I went to give it to the vet, she said it was no good because there had been excrements that had been in contact with the urine. How do I go about explaining to Thumper that she has to do a number 1, but not a number 2...?
Anyway, thanks again a lot for reassuring me. I really appreciate it.