What Causes Headache During Periods?

Hi I am 41 years of age and have 3 young children. Since having children i experience extremely bad headaches on the first few days of my period and nothing I take can relieve the pain. It is constant! Can you tell me why this happens and if there is anything I can take to help me stop these headches? I tried taking 1/2 a 5mg primolut but it irritates me. Please help!!! Demi