Late period, BFN

Hi ladies!
I am really stressing myself out over this. My period is 2 days late (I know --
Not much)... But all tests have been BFN. I have no symptoms other than slight irritability. ... Only thing that's changed is that I'm exercising significantly more than usual because I'm training for a half marathon. Have any of you missed a period because of a change in exercise?? 
Thank you!! :)