It’s honestly sounds like you need to have a different conversation with your GF.

The fact she didn’t believe you, and had to go see for herself means she didn’t / doesn’t have respect or trust you in this situation. That is a red flag you need to address now, because that behaviour won’t go away.

The rest is just mental abuse.

You don’t get mad at your partner when they answer a question you ask honestly.. all that does is breed a relationship where you feel the need to lie. What you do is talk about your feelings, and how someone actions are effecting your. If she felt threatened or worried about how your fantasies makes her feel, then you two need to work together to come up with a solution (and yes, that might be she needs to do some personal growth) . You don’t just freeze out your partner.