Yellowing?

Just noticed how brown your buds are.I wouldn't worry too much about it mate,you must be due to start your flush at any time.It maybe just what the buds have had.

28 is perfect.I guess its shown itself due to it having to transpire more.Enviromental change could cause slight shock.I think it looks like Iron as much as Nitrogen,especially if your using coco.Is it just on the older leaves.You have mobile and imobile elements,I forget which is which but calcium is imobile.It can't take it from the rest of the plant when needed like mobile can.Give cal-mag and make sure everything else is tuned in.You could give it a good flush but make sure to fed it straight away after.

Your ph and ec/tds of your run off.

looks like you have some nitrogen toxicity.. Heat usually affects the shape of the leaves first ie. Curling upward so it's most likely not the 28 Celsius although that is a bit on the high side.. If you RH humidity isn't too low you won't cook your ladies. You've probably gone too heavy on your nutrient formulas.. Your nugs look fairly nicely sized so good work on that end.. But unless you were aiming for short and stout or it's shorter genetics then chances are you have "over salted" and this slows down growth (some strains handle it better then others). Check your run off and TDS.