Okay so I need an expert

Suzie
So I have been tracking my cycle since March after coming of cirazette .  
Period March 6th 3 days 
Period April 9th 3 days 
Not sure on ov dates but app tells me I'm due and I haven't had af but have had bfn today.
I only did the bd 22/4/16 , 25/4/16 and 28/4/16 but was meant to be in the supposed window .
I have been having one sided cramps since Sunday (now Thursday) .
Any thoughts advice or help , is there still a chance I could get bfp