The depo shot is pretty powerful. It's my form of birth control (when i'm not trying to conceive)
it does take your period away (in a lot of people) I went with out a period for 3 years!! (it was great... I'll be back on after this baby lol)
but as far as your test... Don't test to early.
And if you have questions about test results, I found this really cool web site called peeonastick
this lady has every test known to man! And she shows all the pictures of postives, negatives, etc etc...
It should help a lot ;]