Oh, oh!!!  Another topic that I (unfortunately) know very well
about... SCIATICA!!!

Yes, yes, yes!  Skinny people get it!  In fact, mine came b/c I was
VERY active to the point where I was wearing out the "cushions"
between my spine on my back!  (The chiro who looked at my MRI said
pretty much that it's a part of "growing old.")

My survival guide to sciatica:
* Take a week off 'til you're able to at least walk.  Ice your back!
* Stay off ANYTHING high impact. Running and high impact aerobics
are OUT!!!  I recommend the pool at first and then slowly switch to
the elliptical trainer.
* If you run or walk, check your shoes!  If they are older than 6
months, replace them!  The cushion's giving out!
* If you do high impact aerobics, find out if the floors at the
bottom have cushions or if it's just "concrete" under the aerobic
room floors.
* If you do treadmill, find how how often they maintain the padding
on the treadmill.  The shoes will help, but so will this.  (I ended
giving up treadmills in the end for elliptical trainers.)
* A chiropractor or a PT can be your best friend!!!  S/he is there
to help you train to strengthen your legs so you'll get full use of
it again.
* This works for me but I don't know if it'll work for you, but
Glocusamine Chrondroitin seems to help me.  (You can find it in the
vitamin sections of your fave drug store.)   It took about 2-3 months
though before I noticed any difference and 6 months before the pain
is noticably reduced.  (I still take it when my lower back is "talking"
to me.)
* XRay's won't do a THING for you!  An MRI WILL tell the whole story!
(I found other contributing factors to my sciatica.)

Cheers,
Joanna