How did I get pregnant on Nexplanon?

I got it in April 2017. I did not get a period since it got implanted except a random one on February 13th to the 19th. My boyfriend and I have unprotected sex at least 5x a week. I took a pregnancy test today and it was positive. I went to my doctor and it came out positive. How did that even happen?! Did I randomly ovulate after my period or something? The doctor looked at my implant and they said it was inserted correctly and everything. What in the world happened?!?!