Pam: Is there something "new" in the environment? A new baby, new pet, different home, anything that you can trace to about the time this behavior started? Is it hot where you are and cooler under the bed or vice versa? What comes to mind at this point is that you have a toddler in the house that has discovered the dog and the dog doesn't want to be "discovered" by anything that squeals and moves quickly and grabs. Is this a possibility?
Other possibilities include: nesting if she's intact; just looking for a place of her own (you could provide a crate to discourage under the bed, but put it in the bedroom to begin). Remember, if she were in a "wild" environment, she would lay up during rest periods in a cave or under a log, so this may just be her expression of that tendency. It's quiet, dark, protected/protectable -- it may just be her "cave". Is this behavior a real problem or just a nuisance? If the latter, I'd leave it go, if the former, why? I do always recommend a visit to the vet for ruling out medical reasons, has she been hurt recently or gotten spayed? If she's healthy and this is only a nuisance behavior, maybe you don't need to "cure" it? Think about her environment and any changes that have come about or changes in the weather you can associate with the onset of this behavior and you will probably find the reason for it, but if she's happy when she comes out, has bright eyes and eats well, I'd say leave her to it and enjoy her when she emerges.