If the pain is in your stomach, then I suggest laying off it for a day, then going on and taking the Avdil on a full stomach only, because that's what I do when my anti-inflammatories act up on me.
But if it's making your BACK pain worse, then that's very strange and it may be that you're allergic to something in it. Have you taken any non-steroidal anti-inflammatories before and been okay with them? I'm sorry, I don't know what they'd be called where you're from, we don't call it 'Advil' here in New Zealand. But if you're not allergic, then this is VERY strange indeed and I don't know what's the problem. All I know is that if it's causing you pain you probably shouldn't take the stuff!
Advil (ibuprofen) can be extremely hard on the digestive track adn can cause horrible indigestion/stomach/chest pains (it does for me). Are you taking the advil with food? If not, give that a try and see if it makes any difference, but if it doesn't, I would definitely check with your doctor about it - there may be something else they can give you.