UGH what's wrong with my plant?? pictures included

Okay so this plant is 14 wks old. Started off very healthy for about 8 wks. Then I noticed some yellowing/browning leaves on the lower portion of the plant. I posted a thread on here, got a lot of responses that it was spider mites. So I sprayed with neem oil. Which didn't seem to help.

Then I thought it may have been the small 1.5 gal container it was in, so I transplanted on June 9. With the problem only getting worse, I now suspect it may be nutrient deficiency I was feeding it an insoluble fertilizer (FF happy frog all purpose 5-5-5) for 13wks but recently started feeding it a liquid fert, General organics's GROW 4-3-3.
Or maybe it is stress (from the 90degree heat or maybe the transplant) but I dont know. The browning of the leaves is getting worse and is spreading higher up the plant so I'm really not sure what it is at this point.

Does anyone have any ideas what is causing this???

ANY help would be really awesome!

Couple of thoughts ...
does that bucket have good drainage and aeration holes? Your roots could be drowning even if not soaked from not enough air exchange.
The brown burns look like either heat stress or local burns from spills of nutrient.
Finally the plant could stand a bit more N and definitely a dash of Mg.
Some thoughts. cn