I tended to find that when AF was due I was actually still moist and not dry. Usually you go dry when AF is due and with all my pregnancys i stayed moist.
I M/C at 5 weeks and fell straight away, so it is possible to fall again. My cycles were long ones to so my positive didnt show up till quiet late. I think i 'O' on day 20 and i think you normally 'O' around day 15/16. I hope i havent confused you but what im trying to say i guess is its possible to fall preg after a m/c and not all cycles are the same. Mine was a 5 week one not a 4.
Good luck anyhow.