do you think its ok not to tell your girlfriend everthing?

When communicating for two years behind there back .With a married lab partner.Without your girlfriend knowing seems some of you dont understand this was for two years not just in a day here and there.And to make plans to meet to sell a class book.Did anyone ever here of boundries in a relationship to keep it healthy?I dont need to know everything but two years granted it was every few months but still.When it surfaced how would it look?