Sounds to me you had a wet dream a natural release of sexual tension. This is not a relapse as this is natural and out of your control. You will be fine
You won’t lose mental clarity and all because of a nightfall, you might feel somewhat weak and might experience some urges the very next day of nightfall, else it’s normal, everyone on no fap experiences this.
It’s a cleansing process of the older thoughts in mind… just relax and let go, it’s not a relapse, u r becoming stronger…just that after night fall, next day have some nutritious foods and drink lots of water. As long as you stay clean (also intentionally), never doubt the process.