How do I bring up that I hate being touched below the waist

It feels weird writing this and I'm not sure if it's the right place (if not kindly direct me to where it belongs). I'd like to be clear about the title. This isn't a situation where I feel unsafe at all and I just need advice.

When I was around 11-14 I was bullied quite badly to the point of being choked, punched, held in place etc. Another more casual act of them was my ass and around that area would be grabbed as a joke because they knew it made me uncomfortable.

I don't really know why and talking about it does feel like I'm being silly but I've realised that it genuinely does send panic, fear and intense discomfort like nothing else through me when anyone touches me around areas like my ass or thigh.

My girlfriend thinks I'm just really ticklish and finds it funny that I make a huge deal out of it (which isn't her fault at all seeing at I haven't been honest about why). So whenever someone jokingly grabs my thigh or my ass I squirm and keep a distance which everyone finds really funny.

I don't know what to say to people without making them think I'm being overdramatic. I know this is a really odd post but I don't know who to turn to for advice about this.