How can I get rid of this constant back pain?!

I'm fourteen years old and for the past week or so I've been having extremely bad back pains, mostly in my lower back but it's been spreading higher. My neck is also rather sore more often than my back. Anyway, I have no idea what it is but it's gotten to the point that I can't stand up straight and I can't lay in certain positions, mostly in ways that press on my tailbone a bit or I get a dull ache that feels like a joint is about to pop/needs to pop.
Anyway... Here's a little bit of what might be causing it.

~ I am overweight for my age, but I've never had back problems this bad before. And I've been losing weight (about ten pounds now) So I see no reason why sudden weight loss would affect my back.

~I tend to lift things with my back, but most of the strain is on my upper back. I've been working on lifting my knees. The heaviest thing I've lifted with my back lately is my 74 pound dog.

~ I'm a writer/artist and I do most of my writing on the computer and finish colouring in my projects, so I slouch a lot because it's more comfortable.

~ I do take a few medications, but I doubt they have to do with anything. I take pills for my ADD, depression, and heart rate/ essential tremors. Are there any medications relating to those that may cause lower back pain?

~ I have little to no history of back pain, only cramps during my period and aches when I sleep wrong.

To sum things up, it's absolutely agonizing to try to stand up, walk, sit in certain ways, and do simple things like cleaning when the most I have to lift is fifteen pounds (At most). Does anyone have any idea what may be causing this? I'm in so much pain now and I have to go back to school soon.

Please help!