Sadness in a dream is read as feeling given room that daytime may not allow. Tradition takes it as the heart processing a loss or disappointment, gently and in its own time.
Where in your waking life have you met something like this lately?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.