Why does my back hurt?
Attention mods: not asking for medical advice, for entertainment purposes only!
A) Stress
B) Fatigue
C) "That time of the month"
D) I'm Old
E) My mattress sucks
F) I'm being smited (smote?) for my sins
G) Life is suffering
= new reply since forum marked as read
And you probably need a new seat, too. Preferably not pink or red. 
(Let's see if you get this one).
In Google images. Should be right in front of you. 
AOTA
I found the best way to keep my back from hurting (I've had a "bulging" disc, occasional lower back muscle spasms, and sciatica, in the past) is:
a)try to work directly facing what you're doing, feet on the floor, no crossed legs.
b)Sit up straight and 'Suck it in'- your gut that is. You don't have to depend on core strengthening exercises (when abdominal muscles are strong, they help support your lower back) if you use them, to support your upper body.
Don't slouch, or even lean against the back of a chair, at least most of the time, such as sitting at a computer. (Doesn't apply if you go to a movie... the person behind you would get mad. 
or driving, or situations where you have to use the seat back to lean against.)
c) If you lift anything, face it directly and bend your knees (unless you have painful knees) This even applies to reaching into the lower shelves of the refrigerator. Balancing the stress on your lower back while bending forward helps a lot.
If it didn't feel better in a week or two I used to go to a chiropractor. But, doing the above, I have not had any problems for several years.
Good luck, I hope your back feels better!
