Student or Learner
"Okay, that was gross. When were you going to tell me you changed
your entire career path?"
How is this possible? I mean, how can one ask a question about things of the past? I can't understand this question. Could you explain please?
It means that at some time in the past, the person addressed in the dialogue above changed their mind. They did not tell the speaker about this change of mind, but planned to tell the speaker at a later time. However, the speaker found out about the plan before the person addressed could tell them Now the speaker is asking about when the later time was planned for.
The person addressed could answer, for example, "I was going (=I had planned) to tell you about it next week".