"Does it necessarily mean you're pregnant if you missed your period"

Missing your period is often the first sign of pregnancy, and when you know, if you should test or not. Normally it is best to wait about 2 weeks after missed period to do the home test.
You could go in for a doctors blood test now.

There are other reasons, some women often just miss a month sometime. Stress, diet, change in life style can also make you miss (or for it to be very late)

Could you be pregnant? Yes. As to the second part Does it necessarily? No. Having unprotected sex? On the pill?

No, you can miss periods for a number of reasons. If there is a possibility of you being pregnant, try a pregnancy test, following the directions carefully.