I just dreamed about you. This is the perfect tense.
There should be many ways to express it.
For example, the first sentence can also be said:
"I dreamed of you." "You are in my dream."
"I dreamed of you."
As for the second sentence, it is mainly to add some decorative adverbials of time on the basis of the first sentence.
For example, just now, just now, just now.