I am 17 and have a massive problem. I have not had my period for just over 3 months now - I have taken 3 different pregnancy tests and they have all been negative. This has happened before, a year or so ago, where I had no periods for 9 months. Although it seems confusing to you, it is very common for young women, especially those who haven't been having periods for very long, to have gaps of several months between them.
Older women can also sometimes find that their periods stop for a while.
I am not sure what tests your doctor has done but it is likely that they have ruled out anything worrying, and you will probably find that your periods start again soon. If this doesn't happen talk to your doctor again, and ask them why. If you are having unprotected sex then there is always an ongoing risk that you could become pregnant, and you should talk to your doctor about contraception. Very few young women find that when they start their periods it all runs like clockwork from day one. After a year or two things settle down into a regular rhythm, but even then every woman misses the occasional period.