SUBREDDIT: r/relationships
TITLE: Me [23M] with my [21F] 9 months, obvious red flag, when is it acceptable to stop giving benefit of the doubt?
POST: Her first serious relationship, my second.  Celebrated nine months yesterday, day went swell and today was a good follow-up too.  Had a long discussion reflecting on things, end up talking about likes/dislikes that we have encountered.  She mentions an air of jealousy regarding female coworkers (I've explained it's inevitable and make sure to let her know what I'm up to, I choose to do so to put her at ease and reciprocation is nice), and I mentioned one instance where she kept mentioning hanging out with a friend...

Not a big deal to me, I tell her to drive safely and have fun, she however keeps bringing it up throughout the day and before she leaves puts on nicer clothing/gets cleaned up,etc.  When I finally ask who it is her response is "you don't know him", still not a big deal to me, I tell her to enjoy and have fun.  

Today she confesses to having "gotten high" with him (first time in 9 months I've heard that she has this behavior?  not a big deal if she's responsible) but it really took me aback.  She claims she didn't want to tell me or anyone for that matter because she feels I would think down on her and she knows it was bad.  *side note, she parties/goes out and I don't (preference, always encourage her to have fun responsibly) however she mentions she thinks I look down on her for this.  

I feel like her omission to tell the truth is hurtful, however I don't know at what point this is a red flag or her "caring about the relationship and fearing a break-up by telling me".  She also claims she's done nothing with him or anyone for that matter, at this point I don't know whether to trust her, other than that she's never given me reason to doubt her.
  
thanks for taking the time to read this
TL;DR: 
Girlfriend lies by omission about getting high with a "friend" on premise of me "looking down on her" because she knows it's bad