Goodwill Industries

Today I went to a goodwill location at SW 89th Street and Pennsylvania Ave. in OKC, ok.
I had my elderly grandmother with me as well as my 1 year old child.
There were 3 traditional dressing rooms and one handicapped dressing room. I went to the cashier, Treshieva, asked for her to let us in the handicap stall.
The cashier, Treshevia, said no. She said the handicap stall is for handicapped people.
And asked me if I/ my grandmother was handicapped.
I am absolutely appalled at the level of rudeness and complete ignorance of this cashier, Treshevia.
She appeared in her mid-late 40s and appeared of Asian/ Native American descent.
My grandmother, embarrassed from what the cashier said, then refused to even go in a dressing room and try anything on.
It is my understanding that a private business can not question if myself/ or my grandmother has a disability or is "handicapped" and we felt discriminated against.
And to let you know, my grandmother requires physical assistance hence why I asked for that dressing room.
I would like a timely response to this matter. My name is Shelby and my email is: [protected]@gmail.com