Bleeding after sex

Normally after sex I wouldn’t have bleeding, but ever since I took Plan B while on my Nexplanon, I’ve had almost non-stop bleeding since then and at first doctors said it was normal since Plan B can cause bleeding, but it’s been non-stop and is making sex somewhat difficult for my partner and I. I panic and look up my symptoms and think it’s cervical cancer or something but I don’t have pain and I tested for stds and they came back negative, so what could it be??