Need help with a situation that i really don't know how to deal with.

Just spent a weekend away with my girlfriend of 5 months with her friends. Guy neither of us knew was there, he was fine, but i got the impression he was hitting on my girl - she is naturally friendly and flirty anyway. My gf was quite flirty with him ('so, are you single?'), he was asking how long we'd been together she said 'about 5 months, long enough'! etc. It was all done as a joke but she didnt give me much attention. Then, when i get back she texts me to say xyz thought i was a great guy (obviously been reading boyfriend destroyer chat!) Then we get back and she calls me up saying 'i dont know if its sunday night blues or whatever but i just want to check you don't hate me' - obviously callin for reassurance. I didnt know how to react so i thought the best thing was do was play it cool and say of course not, had a great weekend etc.it was great to meet everyone, esp this guy...it was short and slightly awkward - i cut it off first...

Should i be as worried as i am, was that the way to respond? Should i follow it up outling that i was unhappy about it (but i cant do this without looking like an afc). Is it right to be very complimentary about him?

I'm pretty sure my gf wouldnt ever cheat on me but this was a test i didnt deal with properly...

Really need some advice here - thanks!

(really like her, and she puts a lot into the relationship - but i want her not to do that again as it bugged me)

To be honest i don't think you should have talked him up (ever so slightly as you did...) As a PUA, you have the knowledge to realize a boyfriend destroyer when you see one. In your perspective it was obvious he was gaming your girl. AS her boyfriend, seeing she got his number and was texting him.. you have every right to be slightly suspicious..
Firstly, yeah, you shouldnt be jealous. You should trust your girlfriend in that she would respect you enough to know your BOUNDARIES. Everything your ok with and everything you're not. What if you gamed a girl in front of her and number closed? Shit would not be going as smoothly as the situation at hand, would it?

You played it cool bro. Good work. It takes a good amount of confidence to stand strong like you did. What i would have done differently though is i would have made the dude look like a clown. (the dancing monkey of the party) "Yeah he was real entertaining. Did you notice that (Insert DLV )! I liked him because he was goofy, but he had some real lame jokes... bla bla bla." As her boyfriend you have more social proof, therefore what you say has an impact on her thoughts of him. You can Amog him till hes nothing but a funny memory or an inside joke in your relationship.
"Jeez... Are you texting that dancing monkey AGAIN??" You get the idea.

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Some girls i meet are quite savage. Always tryingta grab up on my package. They say i look yummy and they want a taste, but im a human not a sandwich.

Nothing if she cheats, she cheats. It the girls fault if you find out though you should tell him.