Were the Nike trainers you first described actually for running in? Anyway sounds like you've hurt your ankle. Get yourself referred to a physiotherapist immediately, this may take some time via the NHS so get the yellow pages out and go private. Do not run on it. The longer you leave it without therapy the more chance there is of permanent damage.
There is a chance that you've just tweaked it and rest may sort it out. Personally i'd get my ass the the physio.