What Causes Irregularity In Periods?

i'm 24yrs , i'm married,i had my period in may11th( since then ididn't had my period for 2 months, from july 10th onwards some bleeding i can see (i'm not keeping any nappkin or pad for my panty only i can see that 2 when i go for washroom wen i wash i can see thread of red color).i'm not having any pain.i'm not pregnant also in b/w of months i took pregnant test i got negative.in may month alsoi didn't got period i went 2 Gynaecologist she suggested me merrapet tab.with which i got periods after 1 week , after dat again i didn't got periods.kindly , give sum solution. now i want 2 plan for baby. is this irregular peroids causes pregancy or not?