Hi James, I was actually put on the road to diagnosis by muscle spasms in my left torso (probably in the obliques). Every night I would be awakened, screaming in pain as the spasm lifted my body off the bed. It didn't matter if I was lying on the right or left side, the spasms in my left torso would lift me off the bed. I feel your pain, my dear. The doctor gave me prescription muscle relaxants that left me stoned until 1:00pm the day after I took the first dose (about 11:00pm). This was not a good thing as I left for work at 8:00am and was stoned for the first 4 hours I was there!
Anyway, it does make sense that the extra weight would add to the pressure on your spine and the muscles that support it. It's been proved that every extra 10lbs of weight adds 150lbs of load on your knees. If that's what it does to your knees, imagine what it does to the other areas of the body. Swimming or a recumbant bike are probably your best bets for getting this weight off, with the added bonus that your body will be better able to deal with the AS because you'll be using those muscles and joints.
As for, "I used to be extremely skinny and never gain an ounce, btw. grrr. What happened to me? lol"
My dear, you are experiencing a phenomenon that way more men than women experience. It's called:
I could eat anything I wanted my whole life and never gain weight, and then I turned 40! My other name for it is:
Justice for all those women who watched you eating anything you wanted for 25 years and not gaining weight, while they gained weight just for looking at a picture of a cake! The other name for it:
Aging, my dear. 
I think I shall now run from the room to avoid objects hurtling in my general direction. Warm hugs,
