3 periods in one month????

So, I started back on birth control last month. I had a period at the beginning of the month and it lasted 8 days.... then. 7 days later I was bleeding AGAIN. That one only lasted about 5 days, then 8 days later I'm bleeding AGAINNNNN!!! Why?!!! Does anyone know why??? I used to be super regular... one period a month and it lasted for five days. Every. Time. This is driving me crazy.