Sounds more like your body adjusting to the new pills. I would say the negative pregnancy test is accurate where you are two weeks late. It would show up positive by now.
Perhaps your body is just trying to adjust to the change of taking the pills and switching. I don't know if you're stressed, but sometimes that can change cycles too. Most pregnancy tests are very sensitive and should pick up as early as a couple of days of missing your period. I used Clearblue Easy digital w/weeks estimator a few days prior to missing my period and I got a positive and it said 1 - 2 weeks.