pregnancy?

Hi all,
So I'm not sure if I'm just being a bit silly or I should go see the doctor. 
My period ended on the 13/02 I had unprotected sex the next day. 
On the 18/02 I experienced spotting and the worst back pain 
Since then I've been feeling sick and my period is a week late. 
I took a first response pregnancy test and it came back negative but still no period? Should I be worried? Should I go get checked out by my doctor? Or am I just being silly