Talk to Gynecologist on Period Pain

i wanted to ask k kya reason hy periods k 1st day itni weakness hone ki? And also bukhar. Mujhe 1st day of period pehle 6 8 ghante k lie tez bukhar, weakness asi k pora ghr ghom rha hota hy agar uth jao to. Na kch khaya na kch piya jata hy bs soe jati hon laiti rehti hon and alhumdulilah max 8 k bad bikul thek hojati hon. Lower body paon tk extreme pressure feel hota hy 1st day of period. Why? Pls guide