Charlie is right. "I was upset" is the past simple. So you are talking about a specific time in the past. "I had been upset" is the past perfect, which is used when we are talking about the past and then want to talk about an even earlier time in the past. If we consider these two examples: "When John arrived they went out" (past simple). When John arrived at the house they all went out together. This happens at the same time in the past. "When John arrived, they had gone out" (past perfect). The people had already left the house before John arrived. They went out at an earlier time in the past.

I was upset - in the past; for example, i was upset when I did this / at the time. I had been upset, that's the past in the past - to state that you were already were upset before the time you mention. I was upset at 3pm. I had been upset when I saw her at 3pm. That means you were upset before 3pm. I don't think many people say "I had been upset" to be honest.