one good skill to develop for touring is to be able to deal with mechanical issues, so learning the basics of bike mechanics and adjustments is a big plus. Combined with that is also developing the problem solving for things that can happen with your bike, so try some simple observations of your rear wheel and rim to see what it could be.
As said, turning your bike upside down and spinning the rear wheel will allow you to look at teh tire closely, in good light, to see if there is an obvious issue with the tire. Its unlikely a flat spot could be that apparent, you arent a F1 car locking up at 300kph, and the times I have locked up the rears, there was never an actual flat spot created that could be felt riding, certainly not how you described it (the sidewalk description was a good one btw)--but do check any way.
next, using your thumb as a guide, holding your thumb against the chainstay, see if the actual rim has a "hop" in it. It will show up easily if there is one, and at the same time, look to see if the tire itself has a clearly visible "hop"--if so, perhaps the tire shifted a bit due to insufficient pressure, this will be clear by eye.
If it is the rim, it will be significant, as I have a bike rim that has a hop in it, and has for years, but no "sidewalk feel" for me.
If it is the tire, deflate the tube completely, make sure the tire is seated properly and evenly, reinflate and it would cure the improperly seated issue, and take away the "hop".
use common sense, and you should at least be able to whittle down the possibilities, and at the same time, get to know your bike and practice some problem solving skills.
good luck, it should be fairly evident to find what it is.
oh, feel the spokes, squeezing two towards each other, going around the wheel, to see if one is obviously loose or broken. Again, you dont have to know spoke tensions or wheel building, but at least you'll know how the spoke tensions feel and if they are all more or less the same.
ps, driveside spokes are tensioned higher than non ds. this is normal and you'll feel the difference.
ps, not a silly question, just an opportunity to learn some new things. Such is life.