Rotten tomatoes :(

That is tomato blossom rot. Your tomatoes are lacking Calcium.

eggshells or crushed antacid tablets

Don't over fertilize or over water them, you can use mulch to keep them evenly watered. A calcium solution made for blossom-end rot. You can buy the Calcium chloride or calcium nitrate at your garden center and place that in a spray bottle to spray the tomatoes. Hope this helps and good luck with those tomatoes. This has worked for me.

Get a product called Super Sweet. As the poster above said, this is caused by a calcium deficiency. Put 1 cup in each planting hole & mix into the soil. That will solve your problem.

Put a little like around plants