Opk CD 13-18

Candice • 🎀10/2/2000 🎮 2/12/2003 🎀 due 7/23/2019
So I finally purchased an OPK and on my CD 13, 14, 17 and 18 I took the test and got a negative each time. Am I doing something wrong? The calendar said I was in my fertile window but the OPK says otherwise. Today is my CD and ice been having some low abdominal pain all day. AF is due 4/11 and I'm really starting to feel discouraged. Has anyone had this experience?