Seriously irregular periods

Okay so I never had this until I got the bar inserted in my arm for birth control.
I used to get my period once a month around the same time for 5 or so days.
After I got the rod, my period is usually late by 2 weeks, and goes for a week and a half. Then three days later I get another period that goes for about a week.
Can anyone please tell me why this is happening I am desperate to know
I feel like if I go to the doctor he will just tell me it is a side effect and offer to take it out, but I am hopeless with the pill so I don't know if it's easier to just leave it in and put up with the periods.
Please help!