What Causes Random Bleeding After Having Contraceptive Implant?

About a month ago I went on the contraceptive implant      and it's been very good. But I had my first period while being on it about 2 weeks ago and it seemed to be normal.But just after my period ended I had sex and noticed I was still bleeding. I figured it was still my period so I didn't have sex for a few days. But then when I did again I was still bleeding afterwards and even a few hours afterwards. I also noticed I was getting very bad stomach cramps which I've never had before.It's a week on from when this started and the bleeding has got a lot heavier. and my partner has noticed small red spots on his penis.Do you have any idea what could be wrong? I understand you cannot diagnose on the internet but I'd just like a rough idea.