Do rabbits get chin acne, like cats do? I have a cat who has a chronic case, all I can do is clean his chin with human acne wipes every day, but he still has a swollen chin and gets some blackheads. Without the chin cleaning, he had constant abscesses and open, oozing sores. My suggestions are based on my experience with stubborn feline chin acne.
Is your bunny spayed? I'm guessing maybe she "chins" things a lot to mark her scent? They'll do it whether or not they're fixed, but perhaps more if not fixed, due to hormones.
Has there been any change in her environment, like a different kind of bedding? Any other change, like food, treats, toys etc.? Does she have a new cagemate?
Sometimes they get chin irritation from their bowl, it can be helpful to wash the bowl with soap and water once every day or two, or switch to stainless steel because plastics and ceramic glazes can cause irritation. (Ceramics made in China for pets usually contain lead glaze). Will she drink from a bottle? That's another good way to avoid chin irritation from water bowl moisture or the bowl itself.
Sometimes it helps to use a deeper or shallower bowl so their chin doesn't rub against the edge of the bowl.
Do you notice anything that looks like dark granules on her chin? That's a sign of acne/blackheads. Again, I don't know if rabbits can get chin acne.
Could she be rubbing her chin on something out of boredom or habit?
That's all I can think of for now.