It's the nutes, and that is clearly overfeeding. Lower leaves shouldn't be dying off already. That commonly happens to an over-vegged plant and a plant that hits flowering. When they are that young, they shouldn't be losing any leaves yet.

Ease up on the nutes, and only feed 1/2 strength once a week, but for the time being, water with clean water until the burning/yellowing has stopped, then start feeding again.
Nochowderforyou Reviewed by Nochowderforyou on . First Post! - may need expert help Pls Hi guys, Its my first Post ! I looked at other boards but this one has given me much help since i started growing. I got my first 6 plants at 7 weeks now. I grew them from seed and only lost one :) - Thay are a mix of White widow and Bubblicious. I ned your guys expert opinion. ..do they seem Ok to you ? . Few things I worry about is 1. about a week ago small lower foilage leaes turned yellow and died. 2 One of the Bubbles has sprouted about 4 heads ..looks like a tree ! 3. I am getting Rating: 5