Truthfully, while I agree with most of the sentiment, I think the hard "no" will act as a wall between OP and his girlfriend. By blocking her you're essentially admitting you did
something you don't want her to know, especially now you've already revealed part of your past, she'll be working overtime imagining what sort of thing you're hiding. it'll eat at her worse. She's poking at it because of insecurity, any response should therefore reinforce that she has nothing to be concerned about because anything else you say will fuel her paranoia. —
Ruadhan2300 2 hours ago