If you have taken 2 tests and they have both been negative then chances are... your not pregnant.

I am not familiar with YAZ and have only seen it mentioned on here so cant comment on the possible side effects it has.

No pill is every 100% effective at preventing pregnancy but again.. if you are experiencing pregnancy related symptoms they would be your body reacting to the preg hormones in your body and therefore also show up on a test.

you havent said how old you are but women in their teens and early 20s still have fluctuations and your cycles are still settling in to their routines.

best thing you can do is take another test, if its negative then potentially contact your doctor and ask their thoughts.

Oh and no complaints on not using comdoms your choice there but just be sure you are both STD free and only with each other!