You'll probably never really know, but at that early stage, it wouldn't really be a miscarriage per se. You could lose the fertilized egg if the progesterone levels weren't right (this kept my sister from getting pregnant for awhile, and she finally conceived her first son while on progesterone). If you are really concerned, I'd ask your OB/GYN, but I suspect that you wouldn't experience any cramping due to losing a baby at this stage because implantation would just be occuring at this point.
Have you recently gotten off of the pill? It isn't unusual to have heavier periods with different symptoms than you are used to because your hormones are just different and your body is adjusting. I know that personally, if I'm off of the pill, I will have heavier/crampier periods a couple of times a year, and the cramping is always in my back.
Have you been trying for awhile? If so, maybe your OB is the best person to ask...
Susan