Is saying "uff" to parents haram but if you said it because, for example, they confused you so you used "uff" because you were confused, is that also haram?
I don't think it is okay to say "Uff" for your parents in any case. And actually, I think the scenario is a bit different because no one would say "Uff" for expressing their confusion, instead, they may say "Huh?" or "What?", for example.
I think what you are trying to state here (but this is only what I think) is that they were explaining something to you, and you couldn't understand it so you said "Uff" to express your weariness (reluctance to see or experience any more of something). This case would be what the verse has forbidden us to do for our parents:
In Arabic (the original text):
فَلا تَقُلْ لَهُمَا أُفٍّ
Say not to them [so much as], "uff".
"Uff" is the minimum rate of bad sayings which means that whatever is worse than saying "Uff" is prohibited too. Also, the word doesn't indicate itself only but showing disgust, weariness, and boredom toward our parents is included as well.