After I stopped breastfeeding and switched to a combination pill last fall I had some periods that were lasting up to two weeks at a time, very heavy, and I believe I was becoming anemic. I had a hard time keeping my head up at work, was very weak and lethargic, and had to start taking iron supplements (which I recommend if you are just starting periods again--it helped a lot).
You can find a printable menstrual chart online and keep track so if it becomes a concern you have a written history for your doctor. Mine gave me one which had "codes" for the type of flow--fully filled in for heaviest, X for regular, circle for light, and I added a dot for spotting. Mine were crazy. I have never been on the shot, so am not sure for your particular situation, but even when I went off the pill altogether I did not have regular periods before I was pregnant again.