I need a little insight..

So me and my SO have been ttc for about 9 months and I have had positive PTs in the past, in fact the last test I took was about 2 months ago and it was positive. I took a few more tests just because I couldn't believe my eyes. All positive. But then AF showed up. Not very heavy and I had no cramps, so I took another PT and it was negitive. And I just had another period. So we have gone back to ttc. Has this happened to anyone else?