If you don't find any oil anywhere (check frame bits too, oil never just gets one thing wet, it leaves clues), put the bike on the centerstand and, maybe with someone holding onto the top of the seat so you don't tip it over, wiggle the swingarm side to side. See if it has any wiggle in it. If it does, see if another 1/8-1/4 turn tighter on the swingarm bolt changes anything.
Tire pressure/wheel balance/alignment may be a good thing to check too. Also, if you recently adjusted your chain tension it's possible that one of the chain tension adjusters could have moved a little starting this phenomenon when pushing the limits of grip.
Finally, were you leaned over far enough to bottom out? I've had mine bottom on the stand and grab when I was leaned over in a tight turn. Needless to say I now know the difference between the limits of my new tires and the limits of the bike itself.
If none of this finds an obvious culprit, start wiping everything from the swingarm bolt back with a cleaning rag and some soapy water (not the chain though
). This is an old trick I picked up from a buddy who used to be a grand-am chief mechanic. The idea is that when you spend enough time touching and looking at everything in a certain area you start to notice when something's out of place. Plus, when you get done everything's all clean for the next ride.
