What Causes Irregularities In Menstruation?

hi sir i have alway irregular period and in december th periods are not came so that iam worried then going to consult a doctor and she suggested to take 6 tablet for 3 day after eat this tablet i gat period after complete my tablet 3 days then after 2 days i get my period and take yamini pill of taking doctor suggestion so my tablet going on but today i see that bleeding is started when i take this medicine main thing is that that i have sis problem