What Causes Irregular Menstrual Cycles?

I had intercourse the last time 18 weeks ago. Since then I've had my monthly cycle, but not regularly. My HPT's come back negative and I had a blood test done at week 8 and it also came back negative. I have absolutely no symptoms and am not showing physically. I don't have the option of going to the doctor is there any chance at all that I may be pregnant?