What Causes Weakness Before Periods?

My daughter, 25 years of age, complained of extreme weakness right before her period. She is a vegetarian, eats plenty of vegetables complex carbohydrates. Not a true vegetarian, she still eats eggs and fish and cheese. She lives in Charleston South Carolina where the heat index is 110 F at times...I thought she might need to supplement with vitamins that have iron. Can you suggest anything to help her get rid of the weakness?