Why Allah created period of women

The biological process of the menstrual cycle is required for reproduction. If you understand the process, the uterine wall expands to provide an environment for a fertilized egg to grow into a fetus. If there is no fertilized egg to nourish, the lining is flushed for the next cycle.

If you believe in a god that created every detail of the life process, you can wonder if there was a better way to design the reproductive process. One that would be less uncomfortable for women. I can't answer that question.

The only answer is found in Genesis. The Koran like the Christian New Testament, builds on the Old Testament which included Genesis. The menstrual cycle was Eve's punishment for the original sin of eating an apple from the Tree of Life in Eden.