When should I test?

If I were to have gotten pregnant on 2/13, when could I possibly see a positive test? I've taken 3 tests. Negative. No s/sx pregnancy or period, except for tender breasts. My period should have began 2/15. I was supposed to have an iud placed on 2/18, but they wouldnt do it cause my cycle was late. The nurse was so sure I'm pregnant and it's just not showing up yet.. I know the chance is low, but I dont wanna waste anymore money until I might be able to see a positive. I'm 10 days late on period as of today, but the nurse thinks I ovulated late and could have gotten prego on 2/13.