Sunbeam - I just wanted to sneak in and say dint let them dismiss your af conerns. I'm 25 (26 in acouple of weeks) and my cycles have slowly disappeared as yours have over the past two years. Turned out I had endo all over the place, not the worst the fs has seen but still shocked him as no other symptoms. I too wonder how a bfp is possible when ad is so light and short but it has improved since having lap and endo removed.
Anyway I will but out now just wanted to reassure age was not the only answer.
Fingers crossed for all you girls.