I Have Missed My Periods Since 6 Months.I've Had A

I have missed my periods since 6 months.I've had a bloodtest,everything is fine. I am 22 years old and weighing about 70 kg(which is not correct for my height). I had this problem before too when my doctor gave me an injection and prescribed 'Eve Care'. Due to the injection my periods started 7 days later but since 6 months again the same problem(I do not suffer from any other medical problem).What should I do?