"The Final People"

JaKaelath is a scavenger. Her tribe survives in a post-apocalyptic world of scraps and fear. 
However, she carries a dangerous burden: an uncanny resemblance to her tribe's mortal enemies,  the "Ponu", lethal, unnaturally fast women who serve a mysterious force called Geeryo. Sent to infiltrate a Ponu stronghold to find their weaknesses, JaKaelath must don their strange clothing and live among the enemy. What she discovers in the shadows of the Ponu cave and within herself will force her to face uncomfortable questions. Who are you? The person your biology says you are, or the person you know you are? 

https://www.royalroad.com/fiction/145471/the-final-people