It just takes time.
My dog has done the same thing but yeah she isn't a Chihuahua
Being adopted you never know what she would be like. Alot of the time adopted dogs, act very differently to dogs that you buy new born for a few reasons.
1. They might've been brought up a wrong way.
2. There old owner might'nt of cared about the dog and didn't take her outside to go do their business when she needed.
And 3. The dog was basically raised from the time she was born till you got her to go inside.
But yeah when I went to puppy school, not me my dog

I was told to praise her every time she went outside and not in. Unless she went on her mats I was to praise her and eventually she learned to wait at the door till I was there to take her out.
Hope that helps