What Causes A Delayed Menstrual Period?

Dear Madam,good afternoon! I am an engineer and working in UAE and married in Feb 2009. Now I was planning for kid. In August my period got delayed. My Gyneacologist gave me Primolut-Nor and it was regularized on 6th September. She did my D2 blood test which gave a result of normal FSH,LH bat a bit higher prolactin (26). But she told there is no problem and to try for pregnancy. I got my next period on 18th October and thereafter I have not got yet. Again I consulted my doctor on 18th December. She did blood test and confirmed that I am not pregnant. She again gave me Primolut Nor and bromocriptin. It has been 7 days since I have taken my last dose of Primolut nor. But I have not got my period yet. There is no symptom also. Please suggest. i had never such irregular period before. Is there any serious problem?