Why do my legs hurt right before my period?

Pain transmission. The most common issue of pms is fatigue which can lower the stress threhold making discomfort more prominant. The pain distribution would be helpful. Most of the deep nerves run along the back of the legs but some come thru the front. U could have an enlarged urterus affecting the nerves but that is difficult to say without exam. Try 2 aleve (naproxen) 2xper dat 5 days period to the period if ok 4 u.