I experience this as well. Sometimes it feels like time is non-linear, and other periods in my life are so close by that every detail of them is fresh and vivid, like it's happening at the same time as now is. It's usually triggered by feeling alone more so than usual, and looking at old pictures, listening to older music and reliving various memories in my mind. It's a strange paradox when the happiest times in our lives also cause the sharpest pain.