That would be my guess too. The yellowing of the leaves and the overall droopy look of the plant makes me think that. A plant that young usually wouldn't have and nutrint dificiencies at that stage, especially in a good soil. Your soil looks moist, if it always looks like that try decreasing the frequencies of your waterings.my guess is over watering(for the limpness)
the purple may not be a problem, that's less clear
there are 3 plants total one of them is sick, sick one in the original post.they are lookin fine now.
Those are two different plants Anonymuss but just like Grow One said, It's overwatered let it dry out tomorrow and by Monday it will look like your other two. The last thing you should worry about right now is Nutrient deficiency Bro just keep up the good work. Also, No need to Re-pot right now maybe a week before flower would be ideal.they are lookin fine now.
A plant isn't rootbound unless there's roots comin out of the bottom both my plants in 5 gallon buckets have roots coming out but I don't have the space in the closet for two 50 gallon drums LOLAll plants are different, and prefer different conditions, to me it looks root bound, If you repot for more root space the problems wil go away.