Plant started looking like this overnight

New grower here!
The plants have been doing fine through the entire grow until now, idk what’s going on! I just transplanted them about 3 days ago, they seemed to have been doing fine, without any issues. However, when I woke up this morning, they looked like this and I’m freaking out because I’m not sure what this is. I don’t give the plant any nutrients because I grow organically and I make sure I purchase soil that already has all the nutrients In it (Fox farm happy frog soil). Plus this is a fresh batch of soil, and I don’t think it’s a transplant shock because the plants are still growing well. However, this has me worried because I’m not sure what it is and it just keep spreading. I don’t think it’s pest or anything like that because I’ve checked all of the leaves and there are no signs of any pest at all. Any idea what this may be from? My guess is leaf Septoria but I read that it’s uncommon for indoor grows like mine.

Also, I am growing in Fabric pots and the plants are about 10 weeks old. I know they’re small, but that’s because I didn’t have the right lighting at first. They’ve been growing fine without any problems at all until now.