My sisters pregnancy tests!

Rebecca • Mommy to a beautiful baby girl born October 24,2017 💗
Hey ladies so my sister has been taking a bunch of tests lately. She's on IUD mirena. But some tests have positive and some have absolutely nothing. It's like each day it changes. One day it'll be dark, next it'll be light next it'll be completely negative then back to being dark again... she uses the 88 cent ones from Walmart and it's not evap lines or anything. What can cause this? She's had precancerous cells on her cervix many times previously and has to get them removed. Could this be a cause? 
I apologize if this isn't the right group for this!