Hello. Thank you for your question. Please contact the Extension Agent and he will be glad to assist you. I have also included a link to an educational publication.
When the bottom of tomatoes rot it's called "blossom end rot"
The following publication does a good job of explaining the causes and ways to prevent this from occuring. But, a quick answer is not enough calcium getting to the plant and also inconsistent watering (which makes the plant not able to take up calcium well). I encourage you to read further from the link below. It does a much better job explaining "blossom end rot" than I can.