Joint pain after giving birth?

What do we know about joint pain after giving birth? A friend of a friend just had her second baby, and is having nearly intolerable joint pain. This isn't something I experienced, and I've heard woman refer to it in passing, but I guess I thought it was something that lasted for a week or so and then went away.

Did anyone have joint pain? How long did it last? Did you take any supplements that helped any? (I recommend fish oil or flax seed oil, a B-complex, and magnesium for any woman who's given birth in the previous three years anyway.)

Do we know what causes it?

As if new moms don't have enough weird aches and pains to deal with!