yellowing from the bottom. Help!

Don't stress, this is normal at the end of flower. It's often called the fade and some plants will get beautiful fall colours. If that happens in veg or early flower, there is a Nitrogen deficiency, but toward the end like this it is normal and the plant is starting to eat the leaves for the final push for flower development. You want to stop feeding now too as your plant is about to finish up. Normally I give a good top dress as they enter flower to anticipate the bulking of the plant and give it what it needs before it runs out. It is better to anticipate than to react, but that takes a bit of experience.