Hi elaine, I often think of you and wonder how you're getting along!
The official guide is that any bleeding after 12 months without should be investigated and if I were you I would therefore go to the doc and get it checked out. Better safe than sorry!
However, it's a fine line really, 11 months, 12 months, 13 months, none of us are the same, we've all of us experienced peri differently. It's probably a rogue follicle that had tucked itself away and that has been stimulated by the high FSH or it could just be an anovulatory bleed clearing the endometrium after the normal hormonal activity following the final menstrual bleed. Personally I wouldn't be worried but I would still get it checked out!
Hope you are well aside from this blip, maybe you should join us back on the mucus and staining thread, we're still all yapping away!!
Sending you all good wishes xx