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 those 3 days to tell them this information? I know I wouldn't treat my partner with that little care or respect. You need to put some logic into the equation. Like why did she not call her phone company to get her phone replaced? Did she call anyone about this lost phone? Or try the find my phone option that most people have? Does her mother not have a working telephone she could of used in those three days to call you sooner & let you know what's going on? If she does then why the hell were you so unimportant in her mind of people to reassure about her absence? I think there is a lot more to this story than you are getting. There is really no other reason to not go home for 3 days or bother to call your significant other for those 3 days unless you just don't want to go home to be with that person & don't care how it will effect them. She was probably out doing whatever she wanted to make her happy. Being selfish and staying out for 3 days until her conscience started telling her better. There is obviously a problem in your relationship and I suggest some communication. Asking her as well to be honest with you so that you may work on the problem so that it does not worsen or if she is no longer interested in investing in your relationship to let it be known so that your not working on it solo. This is not a solo thing but duo. Taking two. So if she cannot give you the respect of treating you as her partner & at least communicating with you then you should support her decesions to not come home & change the locks the next time she chooses not come home. Help her out by supporting her decision and change the locks.