What Causes Abdominal Cramps With Heavy Bleeding Post Contraceptive Implant?

I got the nexplanon implant about 3 weeks ago, and roughly 5 days ago I began spotting, a couple days ago I noticed I wasnt bleeding as much on the pad but I poured out when I went pee as if it was sitting in the vagina, well I have been bleeding a little more on the pad but still having a lot of pooling blood, I am passing LARGE clots and have been having bad stomach pains. My OB can t see me u til tuesday next week, what should I do?