I'm 23 also had extreme cramps as a teen and long periods and alot of bleeding and was also not sexually active. When I was about 19 I went to three different doctors and they all told me to go a pill called ovral. It really helped, my cramps have reduced tremendously and I only get my period for about 5 days.
sounds a little like endometriosis. google it. if all your doc wants to do is treat your symptoms and won't take steps to find the cause, then i suggest u get a new doc. thing that helps me is exercise and lots of water. i had really bad cramps in my teen years that went away after I started birth control.