That's happened to me twice and it's been super creepy both times. Both times have been with Russian primary care physicians that I saw for annual check ups. When they checked for lumps under my armpits they went "tickle tickle" and it made me feel super uncomfortable.
That being said, a very attractive young medical assistant was inadvertently tickling me when putting suction cups on me for an ekg. That, however, I was very much okay with.