This looks like the disease called Blossom End Rot and is caused by lack of calcium in the soil along with inconsistent watering. Letting the tomatoes dry out then watering too much encourages the problem. A mulch on the soil and consistent water controls this. If you reuse potting soil it could be lacking in calcium the second year adding the lack of calcium. To control add 2 tablespoons Epsom salts to on gallon of water for extra calcium. Then use an all purpose plant food like Osmocote that contains calcium to help control the disease in the future.  Pick off infected fruit and you can still have a fresh start. Keep growing, Marianne Binetti