AF due yesterday

So my AF was due yesterday, I tried to take two tests this morning but they didn't work so thought they was invalid, took a test about half hour ago (5.00pm) uk time, and I got a BFN. My AF is usually on time for me so this is unusual. Would it be because I waited so late in the day to test? I do drink quite abit of coffee. Has anyone had a bfn then get a BFP ?