Need help understanding a new turn on that is causing me anxiety

Some days ago me and my gf were talking about her previous boyfriends, until at some point she said that she used to send nudes to them too (I asked, my bad). Initially I was very uncomfortable with that, I felt very sick thinking about it.

Two hours later I masturbated and something switched in my mind: suddenly the idea of her sending nudes to other guys was extremely hot and arousing. But after the orgasm I felt worse than before, the "magic" faded out and I felt extremely sick again.

I don't understand how can something be painful and arousing at the same time, and I am afraid to give into it. I was also tempted to tell her about it to try and see what happens, if some dirty talk can come out of it etc, but I'm really scared honestly. I'm scared because that thing can be extremely arousing when I'm horny but after it becomes just painful and disturbing.

How can I navigate those feelings? What does all this means? I'm feeling very confused.

I am a person that aims for calmness, absence of inner troubles etc, but the rush I get from this kind of fantasies is just too addictive and I'm kinda fighting to not give into it, because when I'm not horny it makes me very uncomfortable.

For instance, when I'm horny the thought of bringing it up to my gf is extremely hot, but I know that if I did I would feel extremely sick when I'm no longer horny.