Does her fur feel like a normal short haired dog's or is it "different"? Just asking because you mentioned Shar-pei, I personally don't see Pei but I know one dog that was at our rescue that by looking at him he looked all lab. If you touched him you screamed Shar-Pei because of his fur and extra skin (although that could have been hound too, his fur was trademark Pei though).
I'd Pittie because he has that look to him and because they are so common, but ridge back could be there too, i have seen a few around so I guess they are not that rare.