Lack of nitrogen uptake looks like the most likely answer, because they are at the bottom of the plant.
If they were newer leaves at the tips, I'd say probably lack of iron uptake.
Such a deficiency can either be caused by too little of the nutrient in the growing media/water source, or, some other problem causing the plant to be unable to uptake the nutrient.
In the case of iron deficiency, the culprit is more often too high PH, than too low iron levels.
Similarly, nitrogen deficiency can be caused by too low PH, not just too low nitrogen levels.
Do you PH test your tap water? And test it after adding nutes? And test the runoff coming out of your pot?
p.s. Some yellowing like this is normal and even desirable in late flowering. It's a problem worth fixing only if it's happening in the veg stage, too early in flowering, too quickly, or as a result of too low PH.