Irregular period. Please suggest me!

Dear sir/madam,I am newly married person (6 months before), first three months my period came correct time duration(30 days once), after fourth month period delayed so I checked pregnancy test in home(15 days delay) but result came negative, after that I went consultation with doctor Dr.Indhra Nedumaran in Chennai ponneri ,she is also checked pregnancy test with pregnancy kit (this time 20 days period delay) there also result came negative.she is given same vitamin tablets  to take every day night,and again come after  15 day, that time pregnancy tested result came negative.so she's given me periods tablets,I used that tablets 5 days after that period came.now I have same problems my period is missing 5 days,so how can I confirm this is pregnancy or irregular period please suggest me