Is it dying?

No need to worry, cannabis is an amazingly smart plant and she is not dying. What is happening is vital growth elements such as carbs, sugars, amino acids, auxins etc which are "stored" in her older leaves, are being used by the plant to supplement flower growth. Cannabis does this because it is far more efficient to use the elements that are "stored" in her leaves, rather than making them from new during a high energy requiring activity such as flowering. Thankfully you have left all her leaves on, otherwise the plant would have to use energy to make those vital growth elements, plus using energy for flower growth. Had you taken her leaves off, some of her energy would be required for making amino acids, carbs, sugars etc as well as growing her flowers, meaning she would have less energy to grow flowers because some of her energy would be needed to make the sugars, carbs, amino acids etc. This is what the defoliation brigade fail to understand, not only are the leaves photosynthesising and making energy for the plant to grow, they are also "warehouses" for storing the vital growth elements for use during flowering. Therefore, what is happening is a normal process for cannabis during flowering. In future you could keep giving "grow" nutrients for the first weeks of flowering and this will ensure that your plants leaves don't go yellow as early. Congratulations too, your plant looks nice and chunky and I wish I could check it out in person! One other thing, try to pick up/remove any fallen older leaves from the pot and grow space, as leaving them lying around can invite fungal/mould growth. Hope this helps,.... Organoman.