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.
Some women find their periods go on being irregular.
This does not necessarily mean that these women will find it harder to have children, especially as these days there are effective treatments to help. There are some medical reasons why periods can stop for a while or even for good. These include simple things which affect us all, such as stress, emotional upsets, travelling, general illness. Extreme weight loss can also stop periods. Extreme exercise can also cause your periods to stop, usually because it is associated with weight loss below healthy normal levels needed for hormonal controls to work.