I've had very bad cramps since I was 12, and was fortunate that this forced me to
meditate before I even knew that there was such a word as "meditate". However, I can't always be in a quite room lying down. Here's how I do it.
I lay on my back, with hands down by my side, out a little from my body. I try to get in as relaxed position as I can. Then I let my mind float at first just a few inches above my body. Then a foot. Eventually, on good days, I can get out of the building and into a nice place with a pond and a whole lot of sunshine and amazingly soft grass. On the way I frolic in the clouds sometimes. After a while I find that the pain has subsided and I'm ready to come back, taking the same route in reverse. If the cramps have been really bad, I then need to sleep or rest for about 2 hours.
The meditation is not always the same. Sometimes I just go to a quiet non-place. Sometimes all I can do is really concentrate on my breathing so that I have no mental space left to experience pain.
I've spoken to a lot of doctors about this, and the best they can still offer is Advil, (or Naproxen or Anaprox Sodium by prescription). This has some effects beyond pain relief that help ease the actual cramping. The problem for me is that by the time I've taken enough to relieve the cramps I'm really relaxed and no longer in the mood to work.
I've heard a lot of reasons why cramping may occur, and we're all different, so I'm sure we each have our own combination of factors. Among the popular are: tilted uterus, larger clots trying to get out, stress, and small cervix. And, of course, disease can be a cause.
The best thing I ever heard is that having hard cramps means that you are in prime condition to have children. The uterus is exercising for the day. And theoretically, people who have strong cramps should have an easier time delivering children.
Update
I had a surgery in December and found out that I have endometriosis. Well, that explains a lot! The only way to diagnose this is the surgery, and I strongly recommend it to anyone. I just completed six months with no pain. It's a miracle.