Cramping might worry women who think that it might be a sign of impending miscarriage, whereas others are likely to think that their period is starting early. As long as the cramping is light, and combined with light pink or brown spotting, and sometimes abdominal bloating, all should be well and there is not reason to be concerned. Implantation bleeding should literally be light spotting. I'll take the risk of giving too much information, and say that spotting is no different to your normal vaginal discharge, except that it is lined with a bit of blood. When there is more than a few drops, and you are bleeding more heavily, it is possible that your period has really started early, or that you are having a very early miscarriage.
Pelvic pain, severe cramping, clots in your discharge, and persistent bleeding that just does not stop are, when combined, good reasons to get in touch with your doctor or midwife. These symptoms can be worrying even if you are not pregnant, and you might need medical care. On a personal note I will say that, out of my four pregnancies, one of those during which I experienced an implantation bleeding ended in a miscarriage at 10 weeks. Because I was using ovulation tests, I knew that I was ovulating and the implantation bleeding turned up the day after intercourse. I have always wondered whether the spotting I noticed has any connection to the fact that I later miscarried. From a medical point of view, the answer is probably no. I will add that there was absolutely no cramping with the spotting I had. How about you? Did you ever have an implantation bleeding, and do you notice any cramping?