No, don't tick over your engine to warm it up, start the bike, stick your helmet and gloves on, your engine should be idling smoothly at above 2k revs by this point, push your choke in a touch to settle it down and ride away. Take it easy for the couple of miles, or at the very least until the bike is warm enough to tick over with no choke.
On any bike oil circulation is not optimal at tickover, getting oil circulation on a cold engine is essential, and on an air cooled bike for 15 minutes it's going to get way too hot.
As for the OP, if you are worried (you were worried enough to post this thread) then drop the oil again, it doesn't cost much. If all seems OK, then just make sure you are warming the bike up properly, 2 miles is not enough.
Is the bike garaged? If so, is your garage dry or does it suffer from condensation or damp generally?