I have quite a few videos from various producers with the women in them range from nude to fully clothed. I've notice that in a significant number of them that once the tickling gets going the women often get erect nipples (but were not erect when the tickling first started). I mention the nude to clothed to rule out the "the room was cold" argument.
Ticking causes erect nipples?
Ticking causes erect nipples?