Hello there. I am very sorry to hear about your lucid nightmare. I think that it is normal for it to have happened within 8 minutes. Time passes by differently in dreams than they do in real life, though they can also happen within the same amount of time. Some dreams feel like years yet only happen within an hour. And some dreams feel like a few seconds even though minutes have gone by. I would suggest not worrying about it.

Also, next time you have a nightmare and go lucid, try to change the dream in any way you can, or maybe face the nightmare or ask what it wants. That way you can understand it better, whereas if you keep avoiding it, it might keep returning.

Good luck!