Day 40 of my cycle, still no AF

Lydia • 19.. TTC for my first baby.
Hi girls, glow said my AF was due last Tuesday, however my last period was the 5th March, so that means I am on day 40 of my cycle.
I did a pregnancy test last week, and 2 days ago. Both negative.
Really don't know what to think, can anybody help? 
I have got one pregnancy test left, but I feel like it is a waste even trying it with the one being negative 2 days ago...
I have never been this late before, and I have been soooooo tired! And my sex drive has been really calm, I've been too tired to BD!
Thanks xxx