What makes me have painful periods on some months but not others?

Sometimes I get horrible period pains and other times I just get mild pains that mean nothing to me. I was scared this morning when my period started because I was getting ready for work and I didn't want anything to disturb me from being able to perform at work. I thought I was get killer cramps that would make me come back home before my shift ends, but to my surprise..it was so mild. I was happy and thankful that nature had mercy on me today. Last month was dreadful..I was in bed for hours, the pain basically crippled me, I was throwing up..despite taking pain killers. SO WHY SO PAINFUL SOMETIMES BUT NOT OTHERS?