im no doggy pregnancy expert, but could it also have something to do with the weather? I remember being pregnant in summer and it's ruddy horrible- plus I didnt have the added disadvantage of being furry all over!!!
If she is near to term, then yes, this is perfectly normal. Her milk production will be increasing ready for her pups, and she will be conserving all her energy for her birth and initial feeding. Naturally she wants to feel safe and secure, and ready to deliver her pups in a safe place, so her bed is the best place for her to be.