Why Am I Not Getting My Period?

i had my last period in march, after that i had intercourse with my boy friend, we did take safety precautions, however i didnt get preiods in April & in may, i used the pregnancy kit & tested twice once in april & once in may the result was negative. i visited by doctor she gave me gynacet & i used it for two days & its been more than 15 days & i didnt get periods. whats the next solution? am worried about the same, please suggest. if the pregnancy test result shows negative is it confirm that i am not pregnant?