5 year old staffy bitch peeing on my bed

I was wondering if you could help. My 5 year old staffy bitch pees on my bed when unattended. She isn’t allowed in the bedroom because of this, but today I didn’t shut the door properly and she’s done it again. She goes off lead rurally twice a day, has a dog door to get outside when she wants, and has been checked for urinary tract problems by the vet. I don’t understand why she does it? Any ideas? She knows she’s been naughty when I have to strip the bed, as she shakes and tucks her tail under. I feel bad for her, but I don’t know what causes her to do it and my husband is increasingly getting fed up of it