Border Collie poops on my bed!

Every once in a while my Border Collie poops on my bed. And I mean a lot--five or six places! Even more often--every few weeks--she pees on my rug in my bedroom. She is a housebroken 3 year old who goes out several times a day. The vet says she is healthy. We've tried changing foods, but that made no difference. Our other adult dog is not doing this. (I know because he is a male who lifts his leg, but the pee on the rug is a small spot on the floor, and he is too fat to get up on the bed--I've seen him try!) She does not do it when I'm home; she does not normally get up on the bed at all. I will say she is an anxious dog, and when she does poop on the bed it is usually loose, as if she was upset. My adult son occasionally comes to stay at the house for a few weeks, and this may be stressing her out. But obviously I can't tell my son he can't stay! What I cannot figure out is: WHY CLIMB UP ON MY BED TO POOP? I have to either close my bedroom door every time I leave or lock her in her crate. Since the other dog and our two cats roam freely, this seems mean. I hate to admit it, but I am seriously thinking I may have to re-home her. It is just disgusting to get home at night, exhausted, and find poop all over my sheets, blankets and pillows. Ideas?