usually the lower leaves becoming yellow indicates a lack of nutrients, in this case, nitrogen. Even tho you had transplanted them, they look to be healthy, also, the may be over watered if you had showered them heavily, they may need to drain. what fert. if any have you given them? and how old are they (weeks, also seed or clone). thanks
~T-Bone
BTW, she looks to be a dominant sativa, good work so far, also, what strain? research and find out about how they like to be grown, watering cycle and nutrient intake. this will help you grow a better more full plant if you know what they like, if its bag seed, have fun and take notes.