You should have gone out with them on her birthday. You sound like a homebody and she wants some fun and excitement. Nothing really wrong with that except for the secretiveness. I am much more social than my spouse; I just went out with a GF the other night and he stayed home. Its only a big deal if she has needs you aren't meeting and she would like you to. You need to ask her this directly. Pay attention to how she acts, not so much what she says.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh