The japanese for that is 変になる (hen ni naru) which literally means hen = strange/weird/crazy and naru = to become. There's....no expression ot equivalent in English so it's one of those ┑( ̄Д  ̄)┍ moments for translators so you'll end up seeing things like "I'm going crazy" or "I'm becoming weird". If there's a better, less literal way to translate that line PLEASE TELL ME!!!
Actually, I think it's good to go for what it's being used already (the foreignized approach) since it's become so usual already, and experienced readers already understand the terms in context. Hard part would be trying to translate it for casual readers (which are uncommon in this *somewhat-underground* genre). Maybe something like "This feels so weird" would be more natural? It's hard to translate it to make sense without risking changing their meaning or their cultural POV of sex lol
i've always been curious what the actual japanese line is for 'I'm becoming strange', and perhaps if there's a better translation. I see it so often in manga and it's such a weird thing to hear in English.