pictue please.... Normally I would say that it is a nutrient deficiency.. but your plant is to young to have these signs.. something is wrong with your soil.. or more specifically the PH of your soil..

or wait..... wait... does the cup have drainage holes....??? are you over watering.. YES that is it.. that is my final answer.. you have over watering and you have suffocated the young tender roots...

am I correct?

iloveyou