Does she work? A lot on her mind, been happy for the past 3 years, merely forgot the day, date.
I've been with my "fiancé" not partner for 4, he always reminds me of the date I always remind him of "the date" just yesterday I texted him stating, " I love you more than I did when I fell in love with you, miss you " Yep true story.. But I still don't remember the date because, well we cheated, first date was 2hrs but when we met so not really a date, 2nd was 14/1..... so we argue over which was the date.
Ask yourself what is happy, right in this relationship to ask that question sounds as if , there is more here, before her forgetting the date, talk to us, we are actually all ok....