yeah and everyone is different in terms of how their metabolism (and body temp) work as they wake up. A half hour doesn't make any difference to my temp, but an hour would. Your body might do a quick wake up phase or something. It isn't for every one and to get to 18dpo and no Bfp I can TOTALLY understand your devastation. It should have been a bfp (and a chemical if AF shows).
Being the chart stalker I am, did you put that cycle into FF ever?