As has already been stated, the loss of blood is minimal. Also, your body replaces the fluids lost. Sharks are not interested in the type of blood discharged during menstruation.
Birth control and other drugs can be used to alter the timing of your cycle. If you want to pursue these methods, it's best to discuss it with your doctor to determine the best method for you to use. However, most women don't find it to be a problem to dive during their periods.
Using an internal cup (either re-useable, like the "Diva Cup", or disposable, like "Instead" - and there are other brands as well) is something many women divers prefer for long diving days and boat trips or times when a bathroom is not readily accessible immediately after the dive. However, tampons will work, too.
The main thing to consider is how you are feeling. If you feel well enough that day to handle the physical activity involved, then go diving. If headaches, body aches, or other symptoms make you feel too ill to be physically active, then skip it, and dive another time.
