Well...at that moment in time she was thinking about your buddy's dick (That's a huge IOI, toward him, btw

)
You have every right to be a little concerned. But that's just it, jealousy is a normal biological response. As men, we know a girl will take interest in a more dominate or suitable male, we do the same shit with women.
I think you may wanna start playing the field a little more and rediscover your masculinity/dominance with women. Maybe pick up some of that mojo your buddy seems to embody. Maybe lay off the BF/GF shit for a while.