For 3 consecutive cycles if you have irregular periods go for USG pelvis to rule our PCOS ,
Thyroid function test to rule out thyroid disorders & even
complete blood count to rule out anemia...it can be due to stress or harmone imbalance also...u didn't mention ur marital status...sometimes lactating mothers can also get irregular menses due to lactational amenorrhea as well...so avoid worrying & do the investigation & review with reports