My 8 year old, unspayed, very sick bunny had this. There are two possibilities and both have the same solution.
1- Your bunny has engorged mammary glands. They are not filled with milk, rather with a nasty opaque or pink or even red tinged liquid. The pink/red colouration is blood. I am unsure what the opaque is, because I was probably not listening to my vet at that point. It’s a reaction to the hormones in her body.
2- they are mammary tumors. A rabbits mammary tissue go from below their chins (like between their front legs) all the way to their butts basically. It is not recommended to remove it all like we do in humans. Mammary tumors are caused by too much mommy hormones in her body.
If you had any intention of breeding her it is not a good idea anymore. Her ovaries will have too much fat buildup on them at her age. So your solution to both these problems is to get her spayed. If she already has uterine cancer you might be charged an “abnormal uterus fee”. I got quotes ranging from $250-2,200 to fix my bunny. Do not wait. This will get worse and worse and just end up being way more expensive than it ought to have been in the first place.
Make sure you mention your concerns with the nipples to the vet. While they are operating on her for the spay, they can also take out the mammary tumors if that is what is causing your swollen nipples. If the nipples are swollen because of mammary glands, they should dissipate 6 weeks after the operation.
Rabbit uterine cancer is slow moving, but these are the first signs of it. Please don’t wait, you will be putting your baby girl through unnecessary discomfort and pain even thought she may not show it.