Should my english bull terrier be afraid of people?

i have an english bull terrier who is 5 months old. he used to be ok with people like my partners mum and sisters and his friend but now if people come to the house or he is taken where there is people he cowers over, puts his tail between his legs and shakes. is this normal for bullies as i thought they were fine with people? please help how can i make him less weary?