I have never had a chemical pregnancy that I know of but I do know that they are very common.
50/60% of 1st pregnancies end in very early miscarriage, normally the women would never even know that she was even pregnant, but I guess hpt's like first response are changing that.
Reasons for chemical pregnancies are normally caused by chromosomal problems with the fetus,
inadequate uterine lining,
uterine abnormalities
or an infection most cant be prevented.

Snowygirl if I was you I would visit the doctors and get a blood test done, there still may be a chance you are pregnant, surely knowing is better than thinking about it all the time!
Doctors tests can show a pregnancy up from between 7-12 days after conception, go you have nothing to lose!