My Girlfriend (45F) Keeps Giving Me Wedgies (18M)

For context, she is my mom’s former colleague, and we began chatting after I turned 18, so there's nothing inappropriate about our relationship. Over the past six months, she's taken to sneaking up behind me and giving me wedgies, which means she pulls my underwear up into my backside. I've asked her to stop multiple times, but she hasn’t listened. I realize it may sound trivial, but she’s significantly taller and stronger than I am—5’11 compared to my 5’4. What would be an effective way to express that this bothers me?