Hi Starr
I would go for another blood test and even ask for an ultrasound. I've heard of people that have had negative tests but eventually went for a scan to find they were pregnant.
Do you feel pregnant..? Sore boobies, nausea etc?
To find out when/if you ovulate, one way is to buy an ovulation thermometer and to chart your temp for a few months. Another is to get a day 21 progesterone test monthly to see what it comes back with. I was not ovulating regularly but was still able to fall eventually.
Good luck and if you are unsure, try to find doctor that will run some tests for you.