I'm gonna give my abs a rest for a day or two (and focus on my triceps, heehee) and then see how my back feels. If the pain still presist then I'll consult a trainer on my form.
Something I'm also wondering is the proper use of the different size balls. I use the larger ones, so I wonder if the size has anything to do with it? Hmmmmm..
As far at the ball being fully inflated, well, it seems to be so. It's pretty darn hard. But then again, what do I know. Another good question(s) to ask a trainer.