Am I out of line here, is my girlfriend just overreacting?

I understand why you would want it to be a moment that the two of you share. It would have been a big step in your relationship to find out you were expecting and it is quite an intimate moment. But, I think she may have been feeling nervous and wanted the moral support of family and friends. You should empathize with her feelings but she should understand that this is something that the two of you should have found out and then, had she been pregnant, you could have gone together to share the news with family and friends. The initial moment should have taken place between the two of you though. So I agree with you but understand where she is coming from as well.