Just tell her what you said here. That your cycle is late and you are concerned about it and that you have never been sexually active and would like to make sure everything is alright. I am sure it is just your body trying to get regular being that you probably have not been having cycles long enough to get a really good pattern down. My daughter is 14 and has been having her cycles since she was 11. I help her keep track of her cycle days and sometimes she is late which could be due to stress, changes in her activity or diet, etc.