I guess it all depends on the person. One of my best friends was in a similar situation as the original OP. He'd think his girlfriend was seeing someone, or was losing interest in him. He had vented to a friend instead of to her because he was afraid she would see him as jealous, or be turned off. Eventually, it got leaked out to her by a mutual friend of theirs and she was furious. She couldn't believe he wouldn't talk to HER about it instead of to someone else.
Everyone is different. But like I had said above, I wouldn't worry about it. Good luck to you!