I believe this kind of marking has to do with needing stronger leadership in the pack - she feels insecure and marks only that which she has a strong desire to protect. Marking outside of the house is too much responsibility, so she marks your bed, because it's relatively safe and she's least likely to be challenged for doing so.
Strong leadership in obedience training and on walks will help reinforce your "leadership" role. On walks make sure she stays beside you not ahead of you. Don't allow her on the bed with you or on couches. Don't let her jump up on you. Make her sit for her treats and her food. If she is laying in a doorway, make her move, don't step over her. If you are eating around her dinner time, make sure you eat first - not her. All of these are things that occur in the wild, it's all about her respecting the "alpha dogs", once she knows you are the leaders, she won't feel the need to protect certain areas any more, and she will be less likely to pee on them.
If you have strong leadership skills - her issues may be medical - we had a dog that was prone to urinary infections and she would end up peeing in places that normally she would abhor to pee (beds, clothes, shoes and the kitchen floor). If you feel it may be medical, have your dog checked out by your veterinarian.
You might find the following link helpful...
http://www.paw-rescue.org/PAW/PETTIP...ip_Marking.php