First off all,if your baby is on breast fed...then period date may alter or may present with amenorrhoea for few days.
Other causes like stress
,emotional disturbances,physical trauma,anxiety
may present with abscence of mens on date.
Your past cycle was regular and for every 28 days with preg test negative.
So dont worry about missed period.
You have to do following things>
*Maintain healthy diet and weight control
*Maintain personal hygeine
*Do urine test again after 15 days of missed period
*Do a profile of Thyroid
function,Sugar,Lipid,FSH/LH,Routine Urine Exam and a USG if mens does not occurs after 1month.
All thing do under supervision of gynaecologist.