The above are all good possibilities, however they've missed the most obvious possibility. That is that you have some carbon accumulation in one or more chambers from that 38 spl. you were shooting. If so, you may have had one or more cartridgeds that were standing a bit "proud" of the cylinder that were binding on the recoil shield. If so, and if you haven't cleaned the gun yet, you'll probably find some scrape marks on the recoil shield.

Tip, if you ever have any issues when shooting DO NOT CLEAN THE GUN until you've looked it over very carefully. Scrapes and trails left in carbon fouling can provide some vital clues to help in diagnosing the cause to a problem.

Other possibilities not yet mentioned.

Excessive carbon fouling or lead builup on the face of the cylinder. Normally this is only a problem when the Barrel/Cylinder gap is less than 0.004 inch and takes a box or two of ammo downrange to become a problem. Take a look at the face of your cylinder and see if there are any visible "scrapes" in the carbon layer that forms from shooting.

A chunk of loose powder under the extractor star. This will have the same effect as that carbon ring from shooting 38 spl., meaning a fresh load of cartridges won't be fully seated in the cylinder and cause binding on the recoil shield. In this case however the solution is to either blow that chunk of grit out of there or use a toothbrush or similar to brush it out. To avoid this problem in the future make sure to point the barrel straight up when ejecting empty casings, doing this keep any unburned powder remenants in the casings as they are ejected.

That's all I can think of right now. IMO you'll need to spend a bit more time at the range shooting before assuming your revolver needs factory attention. As you can see nearly every single potential problem can be addressed with cleaning or some basic maintenance. BTW, with Sellier & Belloit's reputation for hard primers I would consider primer flowback to be rather unlikely in your model 66, generally that particular problem is associated with Federal primers.