WEIRD CYCLE. when did i conceive?

so today i got my first bfp at 56cd. my last period began on the first of august and lasted till the 5th.. when i was originally late i took several tests and my last one was on cd 38 (i used an early detection test also) which came up a clear bfn. Because of this I presumed my period would arrive yet today I have a bfp and the indicator said I was 3+ weeks? therefore I am confused as to how far along I must be, can anyone help??? also, if its useful at all I experienced spotting precisely 2 weeks ago today. if anyone could help id appreciate it so much!!