Bfns.. Convinced something isn't quite right

Danielle
So we last dtd on 19th Feb.. And I've been testing since Monday.. All bfns
My question is..
When not charting when ttc checking cm/opks etc how long after last having intercourse would you test just to be sure and when would u class yourself out.. Even when AF is non exsistant. 
I have painful sore nipples, had bouts of nausea yesterday and today to extent where I have actually threw up..
I'm convinced I'm pregnant but nothing is suggesting I am via testing.. I'm hoping there's some explanation to this.. With last dtd being 19th it's been 19 days now if I had allegedly ovulated around that time.. But then on the other hand I may not have it could have been later... Question I'm basically asking is I tested this AM neg on cb.. Am I out.. 19 days is a long time right?