I'll second the 'feed whenever s/he'll take it' advice - CJ was almost permanently on the boob it seemed the first couple of months - and my supply has been great.
Also query the use of the shield, get rid of it if she can as bub does seem to dislike it, or if not then make sure to express after the feed so the supply doesn't drop in that breast. Personally, unless it's for inverted nipples or something I'd be thinking shields just prolong the 'getting used to it' part for the nipple and would mean it would stop hurting later rather than sooner.