9 week old strains on gas/BM

We have a 9 week old son (3 weeks corrected age) who weighs just under
13 lbs.  He always puts on quite a show when pu****ng a BM or gas.  It
looks pretty painful!!  He grunts, flails, and cries sometimes.  This
typically occurs within an hour of eating and will last 5-20 minutes.
After he seems content.  He is not constipated and has normal stool
color, composition, and volume.  He was diagnosed with infant reflux
and has rice cereal in his bottle per dr recommendation  (4 tsp to 4oz
breast milk).  He did this grunting etc before the rice cereal
introduction so I'm not convinced that's the reason (we only mix the
rice cereal every few feedings).  But I'm reading that rice cereal
shouldn't be introduced until 4-6 months.  We've asked our dr. several
times about this and she says it's normal, some babies just put on
more of a show.  Are we being paranoid first time parents, or is this
normal?