Why is my dog doing this?

ok so for some reason my dog isn't wanting to go outside. you use to open the door for her to go out and she'd run right out with no problem and stay a little while. now for the last couple months she hasn't wanted to go out. you have to tug her by the collar and walk her out and stand on the steps for her to even go down the stairs. and then she doesn't stay out very long. it isn't even that hot when she goes out. what could be the matter?