Almost 3 Yr Old Suddenly Eating and Pooping Less - Cause for Concern?

[not_telling]

DS is about 2.75 yrs and has been, as far as I can tell, a fairly typical toddler as far as eating habits go - i.e. eats decent breakfast and lunch, has a couple snacks during the day, and usually doesn't eat much at dinner.  He has also been a once-a-day (occasionally twice-a-day) pooper for most of his toddlerhood. 

The past two weeks have been pretty different and I'm trying to figure out why.  He's gone from eating 2-3 "toddler-sized" bowls of cold cereal in the morning to just one and sometimes will barely eat anything at breakfast (this is after eating little to nothing at dinner the previous night).  His appetite for hot cereal (oatmeal or cream of wheat) seems not to have changed, but I only have time to make these a few mornings each week.  His appetite at lunch also seems way down most days.  Sometimes he can't even finish a sandwich (e.g. pb &j or turkey and cheese) made with just one slice of bread...whereas for a long time he would eat that size sandwich and then ask for another and have other things to eat with it, too.  And dinner...still not eating much.  He will ask for snacks (usually something like fresh fruit or dried fruit and nuts or crackers and cheese or yogurt) twice a day usually, mid-morning and after his nap, and I try not to give him so much that he won't be hungry for the next meal.

And he's now going 2-3 days without pooping...which worries me because I wonder is he just pooping less because he's eating less, or is he eating less and pooping less because something's going on with his GI system or some other health-related issue?

The only changes that have happened recently are:

We switched from a potty that sits on the floor to one of those potty seats that sit on the toilet.  He has a stool he uses to climb up on, but the placement of the steps on the stool don't seem to offer the best placement for foot placement for making poops (one step is low so only his toes touch, the other step so high that his legs are almost out in front of him).

And, two weeks ago I started thinking we should offer him more milk to drink during the day (as opposed to just in his cereal in the morning and a small cup before bed).  But, as soon as I started to notice the eating/pooping issues, we eased back on the "extra" milk because I wondered if that might be the culprit.

Is this change in eating/pooping a normal age-related thing or should I be checking in with the Dr.?

[34me]

Can't help you with the pooping habits but two of my three kiddos started eating less around 3.  I always chalked it up to the fact that they grow sooo fast for the first 3 or so years and then kind of have a steady growth, and then wham again....