Is this over-watering? Or have I misdiagnosed?

This is my first grow, so I'm freaking out about everything. I transplanted these four girls 4 days ago. They are 3 weeks from when they broke ground, in fox farm ocean forest. Some of the stems purpled a week ago after a couple nights when I had forgot to set the heater and the temp dipped to 60°F. But other than that it's been a constant 60% humidity and 75°-80°F. I am confident I overwatered around the time of transplant, leaves sat heavy and droopy for about 2 1/2 days. But now the bottom leaves of each plant have significant yellowing.

If its over-watering as far I know there's nothing I can do but hope that there's no root rot. But if this is something else and I've misdiagnosed then I want to get ahead of it. Any help or advice is greatly appreciated.