Your girlfriend left for work 3 days ago didn't come home and said she lost her phone. So she decided to stay at her moms. This has happened to many of us. Maybe not exact situation but something similar. This is what it means.
It means that she has no respect for you or your feelings. You were probably worried right? Had trouble focusing on your day cause your wondering where she is or if she is okay? Now ask yourself would you loose your phone & opt to go to your mother's for 3 days instead of home and not call your partner for...