Am I wrong to be smug?

No.. not really she betrayed you and I guess you are just happy you know now.
If you don't mind me asking, what did she do? Because usually the person would be sad not smug

Told me she was seeing her sister when she actually went for a meal with a lad, so I can't wait to hear her reasons for lying and seeing him.

Oh okay. I think you are just happy to have a bit of power. Listen to her explanation - don't be too quick to attack.. good luck with it all x

I think it is an indicator that you are not bothered about the relationship. Catching people out can be fun but it is not fun if it means you lose someone you cared about.

personally I wouldn't be smug i'd probably be confrontational and angry. Going out with another lad and lying about it would p*** me off soo much lol but I know exactly what you mean by being smug about it all it's just I'd be too frustrated to react like that. Anyway, good luck bro I actually feel for you if she has no explanation for it

I've discovered my gf has been lying to me, she doesn't know I know yet and I'm gonna drop it on her tonight whilst she's out, I really like her and wish this wasn't happening, but because it has, I feel really smug about it and I'm kind of looking forward to the coming argument to see how she tries to justify herself. Ideally we won't breakup, but it's likely that we will and yet I'm still smug about it knowing she isn't pulling the wool over my eyes any more. Is that messed up?

I've had suspicions for a while now and rightly or wrongly I asked her about this and she denied it, so to finally have some form of proof to confront her with has made me happy. I'm extremely bothered by this relationship, I was fully prepared to have my future with this girl and go the distance.

Told me she was seeing her sister when she actually went for a meal with a lad, so I can't wait to hear her reasons for lying and seeing him.

Just to cover all sides here, do you know this guy?Is she friends with him (that youre aware of)

Even if both are true, not sure why she'd lie though.

I think she's had relations with him in the past so it's all the more worrying

Okay. Just wanted to clear that up, as he could be just a friend etc but I get you now!

Yeah that's fine, I'm anxious about it but i need to confront her

Okay. Just wanted to clear that up, as he could be just a friend etc but I get you now!

Even then, why the lying and covering up?

When I am in any serious relationship with someone, I would expect them to be totally upfront and transparent about what they are doing, same applies for me. If the girl lies about such trivial things, how would you expect her to be honest about more difficult issues that are surely likely to pop up further down the line. If I was the OP, I would have serious alarm bells ringing.