You have to realize that life, and other people's conversations are not all about you. You also are not entitled to know all about other people's business. If you happen to overhear part of person's conversation with someone else, that does not entitle you to be briefed on what you missed. At the same time, it does not mean that the people involved in that conversation are slighting you in anyway. For pete's sake! A private conversation is supposed to be PRIVATE! That means that people who were not part of the conversation, are not entitled to know all the details. It is rude to "barge in" on it either during or after the fact. That means that conversations that do not specifically include you are NONE OF YOUR BIZ!. Lighten up on your gf and anyone else you have been pestering. Let it go. If a conversation does not specifically include you, it doesn't concern you.
Something else to consider. If you keep driving the people around you crazy over this, you will drive them away. That includes your gf. Do you really want that, and do you really want to come across as a nosy jerk? That's the way many people look at such behavior.
Sorry for the LOUD parts. That was done to stress some points, not in anger.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau