So, you think shes smoked weed a few times? And it does sound like she was a little mullered. But its up to her if she smokes it...
Sure, you're worried but you were a very regular smoker for a long time so if you've only seen her high (or you've thought she was) a few times, chances are if she had a captain redeye on the go and had the giggles she a pretty novice smoker, considering after a few times using you skip the fun part and go straight to the 'monging out' stage. She's probably just larking around and having a couple joints with friends if she is, there isn't a way you an help someone if they are experimenting with drugs..
You sound like you're both pretty young, and you haven't been together long, I'm sorry if it not what you want to here, but 4 months isn't a long enough time for you to start helping with her using drugs, especially since weed is the least dangerous drug around. I could understand you worrying if you found a crack pipe, but she's just a teenage pissing around with weed. She can either stop by herself, decide she doesnt wanna pay for it, decide that she can't be bothered with it or just decide she doesnt like bieng mullered.
I think if you try making her stop something she is just trying she could just resent you for it, espeially since she hasn't admitted to you that she was blazed. I know if someone tried to tell me too stop I'd resent them for it (Granted I'm not your gf, but you know what I mean)
Butt talk to her about it and see what she says, you can't do amymore, if you tell her to stop she'll pull the 'double standard' card, as you used to smoke it...