Missed one late again!!

I have the iud skyla will have had it a year on July 11th I missed my last period and I'm now late on this current one. me and my boyfriend have an decent amount of sex w/o condoms but he always pulls out before he even starts to ejaculate. with my first missed period I took a pregnancy test twice on the 10th day of missing it and both were negative I don't know how taking one works did I do it too early should I take another one? What brand will definitely tell me if I am or not and when should I take it so I get the best reading? I don't feel like I'm pregnant or have any pregnant symptoms that are common. My dr said that the goal of an iud is to stop having a period but I don't know how to tell those signs other than taka test every time I don't get one. :( any advice anyone