I think it has been overwatered , it is only young and doesn't drink much and can get purple stems because of it, the other options are magnesium deficiency or your environment is too cold at some points in the day/ night unless you have a strain with purple genetics I would check your root pH, water pH (after nutes are added) check temps and make sure the only time you water is when your pot feels light. It is small and not drinking much, too much water could lead to stagnant water and possible root rot..... What strain is it ?