Filling done right?

I had a filling done last week w/ composite.  The tooth didn't hurt until the filling but now hot and cold exposure (i.e. drinks/food/air) make it hurt and the pain can last several hours after the initial agitation.  I saw the dentist today and he said I probably need a root canal and sent me to another dentist.  He fairly much said the filling was done right and that this can just happen.  Really?  Should I look for another dentist vs the 'referral' he sent me to?  Do I really need a root canal or is the next appropriate course of action another filling?