I get my periods every 17 days max every 20 days. I'm always in so much pain?

I have too much pain when I get my periods ! I vomit I faint my legs feel so weak. How come I get so much pain? I'm 23 this year and why do I get them every 17 days or 15 days or max 20 days ? I used to get them every 28 days ! Iv nvr slept with any one so I dnt think it relates to . Plzzz help thanks