Surprised you made it this far with such small pots. The roots really don't grow much past the first few weeks of flower, so I'm thinking that the buffers in the soil ran out (usually after 6-8 weeks in the same soil/pot) and you've been having ph problems and/or the wrong mix of nutes. Judging from the pics it looks way too early for so much yellowing and dying leaves.

Best place to start is getting a digital ph pen and measuring the ph of your feedings and also the run-off. The cheap color drops/strips will not work for this. Can you take some pics of the entire plants and not just the tops?