It might be hormonal adjustments for being off the depo. Generally, your OB will have you come in for a pregnancy test about 3 months after you think your period stopped. Also, it's fairly common for at-home pregnancy tests to be incorrect. For some women, the at-home tests are never going to come out correct.
I suggest going into see your OB in October, 3 months after July, for a pregnancy test just to be sure. If you still do not have your period regularly after that much time and you are not pregnant, then something else may be wrong. The doctor should be looking into that as well.