Has this happened to you.?

So we have been ttc for a few months now, trying mostly when I have my "most fertile days" the past few months and nothing. well now we are not keeping track of my fertile days and my last period was 8/6 and we bd up until the day af came and the day she left, and every other day after... NE ways for the past four days my lower back has been sore, not extream I call it a dull ache and I have had slight cramping. I know it is way to early to test but could I have ovulated right after af??? Has anybody else gotten pregnant that soon after af?