Overwatering...the pots you're using are similar to what I'm using (assuming they are 3 gallons) and I used to water my plants only once a week when they were that size...cut back on the watering and get a moisture meter. One of those you stick in the ground is what I'm using, it has 2 metallic poles you stick in the ground and it displays moisture/ph. Otherwise just use your finger, stick it into the soil about 2-3 inches and if it feels moist, let it sit for another day or 2 and then water well.
Contrary to popular belief, you shouldn't let the soil dry completely (bone dry is not good), but keep it moist. Water circulation is very important and must be kept consistent. I found that my girls were happier and grew better when keeping the soil moist all the time. That, however, doesn't mean overflooding. Let us know how yours recover...you should see a change in about 1-3 days, depending on how much water you gave them the last time.
And by the way, don't feed them more than once a week, and use no more than 1/4 of the strength advised. If they don't get leaf burn tips, then add a little more fertilizer to the next watering. Depending on what nutes you're using, you have to use them every watering (for the low-quality brands), and once every twice or third watering for the best ones. Just check the dosage and keep it lower than they say at first(because your plants are still young), and you should be fine