Pantsed by my friend - how should I react?

Me and my friend are both in our early 20s and we've been doing parkrun in our town every Saturday morning for the last few months. We normally chat to some people every week at the end, but this last week we were talking with some guys who we knew from highschool. I was stood talking when my friend grabbed my shorts and pulled them down right in front of them.. They all saw my underwear and I was pretty embarrassed, my friend seemed to find it funny. Is this a normal thing for friends to do? how can I talk to her about it? also how do I feel less embarrassed about those guys seeing me in my knickers?

Pantsed by my friend - how should I react?
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