I don’t have any personal experience with this- my female bunnies were spayed before I adopted them. However, I would try not to panic right now. I would think that if the lump was anything that has spread, they would have been able to tell while operating. But I’m no expert. I do know that if there is cancer contained only within the uterus, spaying is considered a cure. Are they sending the tissue off to pathology? It seems that blood in urine after spaying could be related to the surgery. Hopefully your vet is very experienced with spaying rabbits, and is right on top of all this.

I wish I knew more, but I have been through scary health things with my bunnies and know how stressful it is! One thing I do try us not to let my mind run away with worst case scenarios. If I start thinking that way, I tell myself to stop, and then focus on doing something useful in the here and now. Petting my rabbit or giving them a nice salad- something they will appreciate ☺. Keep an eye on your rabbit’s temp- when mine get cold, I grab them in a blanket and hold them on my lap, putting a heating pad on low between blanket and bunny, on my lap so I can feel how warm it gets. The vet should have provided something for your rabbit to help with pain too. If not, call and ask. Some vets don’t get how much pain relief helps bunnies recover.

Best of luck and please keep us posted!