I’m confused

Okay so I just had a 12 day period (have nexplanon) that ended on Sunday. I came to my boyfriends house on Tuesday. We had sex or whatever and a day later i’m BACK on my period 🤦🏾‍♀️ But this is not the first time, it seems like i get my period every time I come over literally you guys. In the past two months I haven’t seen him I’ve only had my period once and it was done. I saw him on my birthday in April and guess what? A day after I get there I’m bleeding 🙄 Is this a sign?? Or am I just overreacting? Replyyy