There are a lot of different explanations ... I agree with everyone that the only way to find out for sure is to go to a doctor.
If you've taken that many tests, I wouldn't think it's pregnancy, but I wouldn't rule it out. I had my last period toward the beginning of may but didn't think too much of it as I have eating disordered/substance abuse issues and I figured i'd just lost too much weight to have a regular period. The last week of july, I went into a clinic and they confirmed my pregnancy ... Before then I had taken two home pregnancy tests, one at 6 weeks along and one at 9 weeks and they were both negative. I don't know if my drug use affected my hormones or what but i'm proof that home pregnancy tests definitely aren't 100% reliable.