Bleeding after sex

I'm 19 and assigned female at birth (I'm a trans man). Recently I've put 2 and 2 together and realized that everytime after I have penitrative sex I start my period after. It doesn't matter if I had just gotten off my period a few days prior. Currently I have the nexplanon birth control in my arm. I don't take any medications or testosterone and I don't have any stds. I don't experience any kind of pain during or after intercourse so I'm really at a loss. The bleeding start almost immediately after. Usually when I pee after I'm already getting spotting and by the time I wake up in the morning the "period" has started. To my recollection (which is not great) it has only really started happening in the last year or so and I had my nexplanon inserted 01/01/24 so I've had it for over 2 years now. I haven't made a post on this subreddit before so if you need more information I will provide it when prompted :)