Cereal at three weeks? What do you think?

Ok so I'm a proud mommy of two, my first is three in a half and it seems like an eternity since my last baby. My second is three weeks and weighed at 8lbs 11oz and when we went for her two weeks check up she weighed in at 9' 8', the problem is that for the last week in a half she wants to eat every hr and a half, she formula fed only so shouldn't she be fuller longer? So I called my ped and he suggested cereal! But I've never heard of cereal at three weeks! He said two teaspoons mixed with formula twice a day, haven't done it yet wanted some feedback just for reurance I guess? What do you guys think? Is it too early? I should mention that before I fed her I go through the list, is she cold? Wet? Uncomfortable? Gy? Needs swaddling? But she sucks her pacifier so hard it leaves her a red mark around her mouth, and she wines until she has her bottle right at two hrs later! Help!