Dog becoming fearful all of a sudden?

Hi, I have an English Bull Terrier, just over a year old, spayed *****. Up until now she has been fearless, nothing scares her at all apart from the hoover. But now im starting to notice shes scared of something I cant see, hear or smell. Its started about a week ago, she was cowering, shaking and panting in the landing against the wall. She did this again another time, then one time she was in the understairs cupboard behind the hoover, today she was next to the couch again cowering, shaking and panting, ive just come out from the bath room and she was under the airer, i quietly called her and she came to me head down calmly and sat on my knee for ages being stroked, i got up after a while and she followed me, leaning against me when i stopped, then i sat down in the kitchen and she got on my knee again. She had a nasty fall (about 3ft) tonight out on our walk, landed head first and grazed her nose but was normal and bouncy after. She normally is a happy bouncy dog, but recently this random fear thing is happening. I thought i had connected it to when the oven was on, but the oven isnt on tonight. Also I dont hit her or anything.This only seems to happen in the house, out on walks shes her normal mad happy self, but this is really random. Im going to take her to the vets to get her checked over, just thought id ask on here. Also my other two dogs arent effected at all, they are just normal.