If it flexed on the chainwheel side and you are a heavy rider, it might be enough, in the right gear, to make the chain rub. I know I had a BB on the way out which did this. Replaced BB and rub stopped. I would say it's more likely to be that area than your frame flexing. Try it! Stick your foot on the pedal and shove the bike sideways, see what moves. On mine everything flexes and the rear tyre deforms so it's hard to tell what's frame flex, what's wheel flex and what's tyre flex (jig required). But, if you look carefully at the crank, it 'seems' to bend as well.