It is my understanding that woman actually begin bleeding (menustrating) one day before we actually notice the blood exiting from the vagina. I also experience this and sometimes for several days even, before actually beginning the full flow.
But if you have any concerns you need to talk with your doctor about it. She or he will most likely be able to relieve your mind.
As soon as you start bleeding that is the start of your period, some women just spot for the first day.