Is a lie in confession always a mortal sin

[Penitent91]

I recently went to confession and confessed all my mortal sins sincerely and contritely without purposefully leaving anything out or being too vague. But then the priest asked me a question concerning the reasons why I had committed one particular sin, though I had confessed the sin itself. I panicked here and gave a false answer. I'm terrified that my confession was invalid. Do I add scariliege to my list of sins and do I have to do the whole thing again?

Edited March 14, 2022 by Penitent91
Additional question

[Nihil Obstat]

Possibly, yeah. Definitely ought to be on the safe side on this one. Best to go back to the confessional and do it over again and add the falsehood to the list.

[PhuturePriest]

I would say the subjective nature of this is very important. If, as you say, you panicked at the question and reacted without thinking, then it is very possible your culpability was mitigated especially if you struggle with scrupulosity (scrupulosity comes to mind since you used the word "terrified", and terror concerning valid confessions is usually experienced by the scrupulous as opposed to a "Oh beaver dam, that was invalid, wasn't it?"). I've had situations where I panicked and said something I didn't really mean but would not subjectively say I committed a mortal sin. But objectively, yes -- I would steer towards saying lying within confession is grave matter.