Homeopathic remedy should be chosen on the basis of your sensations. Is the flow copiuos or scanty, do you experience cramps before or during the flow, what colour is the blood, are cloths falling.
If the pain grows stronger with the intensity of the flow, spasmodic, paroxysm, tearing, drawing; worse on cold, better from bending double; if you experience headache in the beginning of the flow - Actea racemosa will help. Apply it in 30 C potency, soluted in water, always stir before drink. The better you feel, the rare you take the remedy.
The most common remedy you can use is Magnesium phosphoricum. Pains stop when the flow increases. It may be taken in 9 or15 C potency. Or you could use the biochemic tissue salt Mag Phos in lower than 12 D potency. The latter may be applied often, to any neuralgic pains.