I've been on Nexplanon for almost 2 years, I have never gotten a period since.

I'd recommend speaking to your doctor as soon as you can. It sounds like you're either going to need an additional drug to stop the bleeding, or a contraception change. I had similar, on the Implant.

I doubt v much you're pregnant, but you can always test to put your mind at rest. This bleeding is often seen on the Implant. It can happen at any point. Penetration can trigger it, if there's blood already waiting to come away.

However, because it's heavy & crampy, I think it's important you get medical advice as soon as you can.

You also need to ask about your shoulder. I can't think of it being connected to the bleeding. I'm not a doctor, though. I was thinking, it could be gallbladder (if it's the right shoulder, especially if it's worse after eating certain foods, like fatty food (dairy, oils etc)), or a muscle strain, for example, so best to mention it to a doctor.