Answer: Unfortunately, based on your description I am not able to tell you exactly what is wrong with your plants. Typically yellowing at the base of the plant indicates overwatering, this can happen with a container plant if the soil is not a well drained mix, or if the drain hole in the bottom of the container becomes clogged, or if the container sits in a saucer that is holding excess water so the soil becomes saturated. Since the plants were fine up until now, I would not expect watering to be the problem. Tomatoes may suffer a a number of potential disease problems that can cause foliage symptoms and it is nearly impossible to diagnose them long distance. So I would suggest you consult with your county extension to try to obtain a firm diagnosis. Good luck with your plants!
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