Why won't my dog go to the bathroom outside?

I adopted a 7 month old dog four days ago. She's very smart. She knew 'Sit' and 'Down' when I got her, I taught her to 'Shake' and 'Fetch' in just one day, and she's starting to understand 'Stay'. She also knows what 'No' and 'Come here' mean. Since she's so smart, I can't understand why she won't go to the bathroom outside. The first day I got her, I took her for a long walk and she pooped outside. I was extremely praising and gave her a treat. The second day she was here, I took her to the 'potty zone' behind my house and she pooped after about 20 minutes. Again, I was very praising. She's hasn't peed outside once since I got her, and she's only pooped outside those 2 times. I haven't caught her in the act inside, as she only goes at night. All three nights since I got her, she has both pooped and peed on the floor. I adopted her from a VERY reputable rescue center, and she'd been there since she was a month old. Her and her two brothers came from a hoarding situation and were almost dead from malnourishment and neglect. It took several months for her and one of her brothers to get healthy again. One of the ladies that worked their also happened to foster my dog as she was getting better. She said that while she had never actually been housebroken, she never had any accidents in the weeks that my dog stayed with her. I know it's only been four days, but it's just odd. When I take her out back to go to the bathroom, I repeat 'Go potty, Dewey' to her. When I wake up in the morning, I take her to where she had an accident and tell her 'Go potty outside!' then immediately take her out. I'm not sure what else to do. Any suggestions, tips, or ideas would be very, very appreciated! Thank you!