Sentences beginning with So + Auxiliary Verb (do/does/is/are/did/has/had/ and many other "helper verbs") = "The other person did (does, is, etc.) the same thing". I just did exercise 14. So did you! (You just finished exercise 14 too!)

Examples:
You are on the internet now. So am I (me too! I am also on the internet).
I am working on this exercise now. So are you (you too! You are also working on this exercise).
Dina went to the gym yesterday. So did Danny (Danny went to the gym too).
She watched TV yesterday. So did he (Danny watched TV also).
Dina went to school yesterday. (Danny went to school yesterday also).