What Could Cause Missed Periods With HPT Negative?

Hello Dr....I'm 26 years old.......trying to conceive for past 4 months....My last period on 11th feb....Normally my cycle days is 30-35....I taken urine pregnancy test on 22nd march the result was -ve and undergone ultrasound scan both the overies are normal...And again taken test on 30th march the report wass -ve...still not had my period.....Iam taking folic acid tab daily in the morning.....Past 2 years iam suffering from goitre....i don't know what to do....pls help me dr........