Answer: It is difficult to identify the problem based on your description, so there are several possibilities. The first is fairly simple in that over-watering can cause the lower leaves to yellow and drop and it can contribute to leaf roll which occurs during very wet seasons. Tomatoes need an evenly moist but not soggy soil, so it is important that your pots have drainage holes in the bottom and that you use a soil mix which allows for adequate drainage. To judge if your plants need water, stick your finger in the soil and see if it is dry for about the top inch. If so, then water. If not, check again the next day.
Other possibilities are less pleasant. Tomatoes are subject to a number of wilts; two common ones with similar symptoms are fusarium (F) and verticillium (V). Heavy fertilization and excess moisture can contribute to their growth. Both can usually be avoided by buying resistant varieties (The seed packets or plant labels say for example " VFNT resistant"; be sure to select a variety with at least resistance to V and F) and always using clean soil, pots and tools. Fusarium can be introduced with the tomato seed itself, and both of these can be transmitted from the supplier of transplants. I hope you've just been watering too much!
Q&A Library Searching Tips