Why is my English Mastiff Submissively Urinating?

Does anyone have any idea as to why my 7 month old English Mastiff who is completely potty trained is frequently urinating and pooing in my house. When I tell her to go outside or in her crate because we are leaving, she gets scared and does her business all over my house. It's not like she's trying to do it, she pooed all over herself while sitting down. Why is she so scared?