"Periods tend to be short and scanty on pills, and you may see no fresh blood at all. A drop of blood or a brown smudge on your tampon or underwear is considered a period. This is because combined estrogen and progestin birth control pills suppress the formation of uterine tissue. Therefore there is very little tissue to slough each month. The scant or absent period is not due to blockage or pregnancy.
-If you have not missed any pills and you miss one period without any signs of pregnancy, pregnancy is unlikely. Do a home pregnancy test or call the doctor if you are worried.
-If you forgot one or more pills and miss a period, run a home pregnancy test or contact your doctor about a pregnancy test.
-If you miss two periods in a row and feel pregnant or if you miss three periods in a row, contact the doctor for an examination, even if you took your pills every day and even if a home pregnancy test is negative."

In other words, a missed period could be anything; in fact, it could be nothing. It could also be a more serious medical condition.
Take a HPT, but since you always take your pills on time, there is a less than 1% chance you are pregnant.
If you continue to miss your periods, I'd suggest you see a doctor.