The levels do go up the further you go along in the pregnancy. It is possible for them to be low enough to not be picked up on a HPT if you are less than 6 or so weeks.
If you O'd a week later than you normally would you might only be around 4 weeks pregnant now, so would explain it not picking up enough HCG.
Thats why I suggested waiting a few days. Coz if you are pregnant the levels should go up enough to be detected.
It could be a sign of etopic as well, so maybe get in to see your gp. I wouldn't stress too much just yet though