Since you have always had regular periods, the first thing you should suspect is pregnancy
. As your HPT is negative, you can either wait for a few days before repeating the test or get an appointment with your doctor for further assessment. Protection in a cycle from birth control pills depends on the timing of the pill. Another possible reason for your irregular periods
could be a temporary hormonal
imbalance that could have various reasons the most common ones being stress and overweight
. Please get further suggestions from your doctor. Good luck.