I've changed my security questions, but when I try to make a purchase it still asks for my OLD questions ?

The new questions show on your account on http://appleid.apple.com ? If they do then try logging out and back into your account on your phone (assuming that is where you are trying to purchase from) and see if the new questions then show on it.

And you've logged out (tapped on your id at the bottom of the Music tab in the iTunes store app) and back into your account on your phone ? You could also try closing the iTunes store app via the phone's taskbar and see if you get the new questions when you re-open it