What Causes Irregular Periods In 15 Year Old Girl?

my daughter is 15 yr old who got her first period in february 2008.till 6 months back her periods are regular.after april 12th she had her period on 28th june after using primolut-N twice a day for 5 days on the suggestion of a gyn.spl doctor.and again two months passed.doctor suggested the same medicine and she got her period on 2nd september. its 2 months since she had her last period. she is in her plus 1 class. shall we go for scanning..or some regular medication for some time