What Could It Be If Missed Periods And Pregnancy Test Is Negative?

hello sir, am a girl of 20 years...i had my last period in the month of september last month i missed my periods n i took a pregnancy test also twice but it came to be negative........nw am very much worried as to whats the problem behind it.......and since last 2 months am also havning very frequent burping problem....please help me out becoz till now i hav not consulted any doctor so plz help me out.