Well, it could be that it is her friends and her who don't want you to come for a girls night out... it would I suppose seem odd that you would be the only guy coming along with her and her girlfriends for a "Girls night"
However, I think it is curious why she would say that she was all okay with the two of you doing more things together with other people but insist that a bar/club would be out of the question

I guess if it were me and my guy told me that, I would wonder as well what it is exactly he didn't want me to see if we did this activity together.
Possible for you to discuss this with her a little more? Maybe the two of you come to some compromise.. perhaps set aside days that are just for the two of you and days that the two of you will do things with both sets of friends?
If nothing else, let her know that you miss spending time with her.. that you understand that she has a busy schedule but you would like some of her time as well.
Good Luck