Hi Jackie. Welcome to NGA!
I agree with all of Robyn's and Rita's points.
First make sure that the container has lots of drain holes and they are not plugged, and water runs out a little while afteryou water them.
1. if the potting mix has dried out and shrunk, "watering" might actually only let the water flow AROUND the potting mix without wetting it. If that's happening, the plant will also die of thirst, so that's probably not it.
2. If the drain holes clog up or there are too few or they are too small, OR the potting mix is too fine and holds too much water to let any air in, the mix would usually be soggy and the roots would be dieing and rotting. It's probably not that unless the plants start getting much worse. But if the soil IS soggy and not well aerated, you will have a variety of problems including root rot and salinization of the potting mix (salts accumulate if no water flows THROUGH and out)..
3. If nothing else was wrong except the container was too small for that variety, you would be likely to see exactly this: lower leaves yellowing. Try a soluble fertilizer like Miracl-Gro, or if there is a fast-acting high-N organic alternative, try that. Since the plant can't make as many roots as it needs in a small container, make sure the plant CAN still get enough minerals from that small container.
I think that 3-5 gallons is the smallest you would want for tomatoes, with determinate cherry varieties OK with 3 gallons, but mainstream indeterminate tomatoes would want at least 5 gallons.
4. if nothing else is wrong AND your window box is big enough, maybe the soil in it has too little nitrogen (and maybe not enough of some other plant nutrients). Low-N looks EXACTLY like you describe: yellowing lower leaves. (But other things could cause that, too.)
Again, try soluble chemical fertilizer or an organic alternative (if any of those are fast-acting enough and have enough N).
The thing to be careful of with soluble fertilizer is that it IS potent. It's very easy to use too much, and plants often just die when you do that. So read the instructions and start out with about half that much, applied once per week until they get better or die.
And remember that water "should" come out the drain holes several minutes after you water heavily. That carries away excess nutrients and salts, preventing saline buildup that can kill plants. When I say "water should come out", I really mean that the potting mix "should" be open enough to allow drainage.
Water needs to drain out so air can diffuse in.
Air diffuses literally 10,000 times faster through air-filled voids and pores, compared to diffusion rates through water-filled voids and pores. When the mix is so fine-textured that most voids and pores aren't much bigger than a "capillary distance", most pores and voids grab any available water and hold it in a firm capillary grip. Now air is excluded until the plant drinks almost all the water in the container. Roots quickly die and rot.
When the mix is coarse-textured enough that many gaps between solid particles are BIGGER than a capillary distance, gravity can pull some water OUT of those bigger voids. Now part of those voids are air-filled, and oxygen can diffuse through them 10,000 times faster. Roots get oxygen and live.
