I went and saw the movie Avengers: Endgame without my girlfriend and she doesn't know I did.
Who did you watch Avengers: Endgame with?
I went with my friend to the cinema.
While it is okay to see a movie with a friend, you should have let your girlfriend know about your plans so that she could have come along if you wanted to. It's important to include a romantic partner in your plans or he or she will feel left out and think that you do not care and your relationship will suffer. You should be honest with your girlfriend and explain that you already watched the movie if asked.
That makes sense.
It's good that you can understand the importance of honesty in your relationship with your gf. Just be honest instead of sneaking around to go to the movies and anything can be worked out, but if you don't you end up with unnecessary conflict.