whether I ought to be worried about this type of flexing
Well, I guess that depends. IF it's wing flex I'd be concerned. But that hasn't been determined. Can you easily flex the wing by hand? Only the right side? Have you cartwheeled it, or had some other crash that may have weakened the structure?
Were you flying extreme maneuvers before seeing this? (Hard, fast rolls, fast dives with quick up elevator, or full power loops.)
It may have been a bad batch of Monocote. Some of there more recent stuff seems to have been reformulated, and in my opinion is garbage. Requires much higher temps to adhere and shrink.
I've also heard of Monokote becoming brittle over time, but usually much longer periods than you're talking about.
Dennis-