Laying down during training?

We adopted a young dog (40lb bully mix, about 1yr old) about a month and a half ago and last week I decided it was a good time to get started with actively training her. Last year, we hired a trainer for a very misbehaving and aggressive dog so I figured I'd use some of the techniques he taught us with her for the basic training. Shes overall really well behaved, but I want to teach her down, stay, and to ignore other stimulants on walks. When I'm walking her with my two other dogs, she acts great and walks right by my side, never tugs or anything. Recently when I started to walk her by herself to try to train those commands, she completely shuts down. She'll lay on the ground and stay and the only thing I can do to get her to get off the floor is physically carry her home, she will not budge no matter what. What the heck is going on? I have never experienced anything like this before. I noticed the word "heel" usually sets this off, and Its almost as if she was previously trained down and stay but I have no clue how to proceed here. Any help would be appreciated! She learned sit and paw very quickly with treats, but I'd prefer not to teach her everything with treats