Hello and welcome to HCM,
Amenorrhea ia most commonly caused by pregnancy.
A history of unprotected sex further increases the possibility of pregnancy.
A urine pregnancy test should be done to confirm or rule out pregnancy.
Since, one test is negative you can take up another pregnancy test.
If pregnancy test again turns out to be negative, then pregnancy is ruled out.
In that case some other reason has to be found out to find the cause of missed periods.
Irregular menstrual cycles are common in healthy mentsruating women.
A consultation with your gynecolgist will be required if the amenorrhea continues for couple of months.

Thanks and take care
Dr Shailja P Wahal