Yellow leaves help????

I apologize for resurrecting this thread, but instead of starting another one, I thought it prudent to just tag onto the end as my question is related.

I'm curious about my leaves. The plants are at least 3 months old and have recently begun to flower (two or three weeks ago tops). I've been feeding with Dyna-Gro Foliage Pro and during veg, I added a dose of Fish emulsion to the solution. It's not nutrient burn, because no other leaves are showing any signs. It's probably not the sprays I use, for the same reason. A few of the larger fan leaves are turning completely yellow. I attempt to pull them off and am surprised that they don't fall right off like unhealthy leaves normally would. They are about half way up the plant, so they aren't the oldest leaves. The lower leaves on my broccoli plants will turn yellow and die as the plant ages, but my MJ plants don't seem to be doing this either. There are no spots and no crispy areas.... just a solid yellow, though otherwise healthy looking leaf.

Any thoughts? I already removed them leaves before I thought about taking photos... sorry.

Also, I spray semi-weekly with insecticide and fungicide and see no signs of either on my plants.