Irregular periods and really bad cramps

I'm 16 and I get really bad cramps every time my period comes to the point where I'm screaming in pain and cry I have been irregular since I started my first period at almost 15 I get two periods a month sometimes and sometimes I don't get one at all I am sexually active but never been pregnant and don't plan on but any advice on what this irregular thing about I'm scared I won't be able to have kids