Implantation bleeding can be one of the earliest pregnancy symptoms. About 6-12 days after conception, the embryo implants itself into the uterine wall. Some women will experience spotting as well as some cramping.
If you have a positive pregnancy test and continue to have cramping and bleeding, it is highly recommended that you go to the emergency room to rule out an ectopic pregnancy.
Other Explanations: Actual menstruation, altered menstruation, changes in birth control pill, infection, or abrasion from intercourse.