Why do i want my girlfriend to have sex with someone else?

I am 21 with a girlfriend who is 19. We have been together for 5 years. She has never even kissed someone before me and I took her virginity. She looks like Marisa Miller if Marisa Miller was 19. She is not slutty at all, doesn't flirt with any guys and she is very very loyal to me. We love each other and will be together forever, everything is perfect. Our sex life is ridiculously awesome. Toys, outfits, dirty talk, everything you can think of we do. She would never cheat on me, believe me when I say that. Lately I have been thinking about her ******* someone else and for some reason it turns me on sooo much. I feel jealous, hurt, and mad when I think about it but for some reason I get so aroused. I told her about this and she thought I was crazy. We ****** right after we talked about it and she was so wet so I new it turned her on. We then went on chatroulette and she did what guys told her to do, strip and played with her self the whole deal. I watched by screen sharing our computers and I was in the next room masturbating. I can't explain this feeling. I am on edge thinking about it constantly but I am so mad at the same time. So far we have just been doing online stuff using chatroulette. She went away this weekend and I thought for some reason it would be so hott for her to send me a picture with some other guys cum in her mouth and on her tits. Remember she has never had sex with anyone but me so why would I want this? She didn't do it and she tells me she doesn't want too but the way she smiles and says stuff sometimes I know she wants too. For example she told me that she can't stop thinking about striping and playing with guys on chatroulette and then she said she was kidding but she's not, I can tell. I can't stop thinking about her getting ****** in all positions from someone she thought was really hott and sucking a big **** and taking it all. It makes me so aroused I do not know what to do. Should I let this happen? Im so scared about my feelings because they are all over the map yet this is the best feeling of my life when I think about this. I literally can't stop shaking and want to masturbate right away when I think about it. And when I climax it is better then I have ever felt. Is this normal?