Are your tomatoes planted outdoors? They could be still adjusting to the colder nights if they came from growing in a warmer environment. Yellowing of lower leaves can also be just a sign of natural maturity as the plant loses the first set of leaves and puts on new growth. Another reason could be too much water so feel the soil before you water your tomato plants to keep them just barely moist. Once summer arrives and the weather heats up I suspect your yellow leaves will be just a distant memory as the plants make blossoms and new fruit begins to form. Keep growing, Marianne Binetti