I don't usually weigh in on the personal questions, so I hope this is useful....

I think your reaction is normal. Whether there is something going on or not, it sure looks suspicious. I am not surprised you don't like this situation.

Go back to the beginning:

so, it all starts out, that my girlfriend (she wasn't my gf at the time) likes my best friend, but he doesn't find her attractive....

I think it is "new girlfriend time." This one is not treating you good enough. I know it hurts to look for a new one, but she just got here and already she is causing you problems. A new romance should be problem-free for at least 3 months (like a walkman, 90 day parts and labor). my two cents. :)