Jchap's right about the yellowing in later flowering, it's normal. You want the N levels in the plants to fall later in flowering to produce a better product. Flowering fertilizers are much lower in N than a vegetative, causing the plants to consume the N from the leaves, hence the yellowing.

You need to make sure they are not yellowing too much, too early in flowering. Since it is only the bottom ones now, you are in good shape if you only have a few weeks remaining in flower. If you have more than the three weeks that jchaps suggested, you may need some supplemental N to keep them going.