yellowing on lower leaves.

As long as it is just the lower leaves, you are fine. In Flower, your buds will pull nutes out of the lower leaves as they start to fatten up. As you get closer to harvest, you will notice more of the leaves turning yellow. It is the normal process as long as you are not pushing to many Nutes at her.

If they're right at the bottom and you're positive you're feeding well and at the correct ph level then they are just not getting enough light down there which is normal indoors for most people