Why Do I Have Brown Spotting After Taking Postinor For Irregular Period?

Hi, I am 20 years and weigh 79kgs. I have been having irregular periods this year. For example i didnt get my period in February. I had sex on 4th March and took postinor 2. I later got my period on 12th. I had sex again on 15 which my last day and took postinor 2. I later had brown spotting similar to periods on 22nd to 24th. Until now i have not gotten my periods. What are the chances i could be pregnant? Or am I pregnant?