Is she spayed? Un-spayed rabbits are very likelyto mark their territory. At the age of 6, I wouldn't trust her to justany vet, but a rabbit-savvy vet should be able to do bloodwork &otherwise determine if the risk of surgery outweighs the benefit ofspaying.
As has already been mentioned, has your bun been litter-trained beforerecently? Has anything changed before she started soiling your bed?Rabbits are very sensitive to change, & if you've moved, or addedother animals to your household, or gotten a new bed, or possibly evengotten new sheets, that may be enough to make your bun feel as thoughshe needs to "mark" her territory.
You don't actually say whether or not you've had your bun for most ofher life, or if you've adopted an adult bun fairly recently. If you'vejust brought your bun into your household, it's very normal for her tobe marking "her territory", especially if she's unspayed. Evenspayed/neutered rabbits, though, often find soft spots like beds,beanbags, upholstered chairs, etc, to be irresistable... my Meat Bunny,f'rinstance, has had practically perfect litterbox habits... exceptwhen she had her own "bedroom", in which case Meat veiwed the futon asan extention of her litterbox.
Oh! Here's a thought! Have you recently added another human to yourhousehold? Your bun may feel particularly threatened if there's anotherperson sharing your bed... hence the need to mark her territory!
If you can't figure out why your bun is soiling the bed & fix theproblem, I highly recommend making sure that the bun can't get to thebed. In my case, I didn't want to move Meat out of "her" room, butsince she's been in her own immense cage in another part of the house,she hasn't had a single accident. Of course, everybody's situation isunique... just ask yourself if it takes more time to change/laundertowels & bedclothes, or to spend quality time specifically withyour rabbit as an individual?