Unfortunately all this rain & then hot sun tends to make tomatoes crack. Heirloom tomatoes are very thin skinned, and when the sun hits a wet tomato, it can cause the skin to split.
One thing that you can do to help prevent this is pick them a little bit unripe. Look for a slight changing of colour at the bottom or throughout the tomato, this should indicate that it is already ripening inside, and will be delicious to eat in a few days.
Remember, cracked tomatoes does not affect the taste! You can still eat & enjoy the fruit, and if it is starting to rot, you can either simply cut around that part of the tomato or, you can always collect seed from it!!!