Thanks to mis-"managed care", today, diagnosis is more tricky:

You don't see the same doctor all the time.  Providers aren't
held responsible for looking at your separate visits as part of a
whole person, so no one connects different symptoms that
come and go, and that affect different body parts.

There's can be such a delay from when you feel bad until you
get seen, the fighting has sometimes stopped.  Then, the
doctor sees no symptoms, and what do you think s/he thinks?
Lupus Mechanics

(as explained by Jo Beth)
Take me back!
Little tiny blood vessels can get clogged up,
causing lack of oxygen, leading to pain and,
sometimes, bone or organ death, or nerve
and brain weirdness.
Pathways for the fluids that lubricate your
joints can get clogged up, leading to
swelling, pain and stiffness.

Some days
you  don't.
Pathways that move fluids in and out of
cells can clog up, causing too much fluid in
some places and not enough in others.  
Or vice-versa, the very next day.  

Some days  
you feel
like a nut.

Lupus can drive your body,
and you,
CRAZY.
Think of two guys who keep breaking into fights in hallways.  
They don't kill each other, but they're laying around all
swollen up afterward, and everything breakable around them is
smashed.  It's hard for anything to get through the hallway.  
That's what lupus does--the damage is more from the waste
products of the body battles than from the battles themselves.

Now, touch a part of your body and imagine that it is that
hallway. Basically, any part of your body can be affected.

Diagnosis can be tricky, 'cause the boys move their fights
around, and don't fight all the time.