Answer: It sounds like your tomatoes might have tomato fruitworms or armyworms. Both of these caterpillars burrow into the fruit and cause it to rot. If you want to try to identify the pest, you could cut open some of the affected fruits and see if you can find the caterpillars. The fruitworm is yellow to brown in color and about 1-1/2 inches long. The armyworm has a black head and an inverted white "Y" on the front.
Both caterpillars can be controlled with Bacillus thuriengensis, which is a bacteria that's spread on the plant in a liquid or powder form. The bacteria invades the caterpillars and kills them. Good luck!
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