If someone is mentally unstable would they act like this?

If someone goes from nice to mean in a split second does this mean that they have a metal issue. If they go from calm to irate in a split second. I cant explain it, its going okay, then all of a sudden they snap. I havent been there in person when they have snapped, but just wondering. This person was like all into me and wanted to be with me. Now he is acting weird and snapped so hard on me that I think he would of hit me if I was there with him in person. This person knows were I live and where I work even what time I come home and leave for work early. Should I be concerned?