4 days of positive OPKs?!

Al
Ladies, is it normal to get positive OPKs for 4 days? I got my first positive OPK on CD15 in the morning, I just test again CD18 at 3:00 PM. Help? Is this normal? This hasn't happened the last two months.. Sorry for the smeared writing on the OPKs, my pen always smears. The last test is from today CD18.