My period 4days late ?? But why

So i was so post to get my period 4 days ago. And then just suddenly i got it whats going on i just don't understand why it was so late I've been trying to conceive for 6-12 months and hadn't had luck. Last month i had spotting 2 days before my period was so post to start and the first day of and that was it enough for a panti liner. Anyone have some info why its been so messed up lately