Can't say for certain but either blossom end rot (BER) or sun burn/scorch.

In any event, fruit is likely rooted at least in part.

Blossom end rot is usually caused by uneven watering. Plant cells need a steady supply of calcium when they are forming, and if the soil dries out and they are not getting calcium, cells at the bottom of the plant start to collapse.

Try to keep the soil evenly moist. Mulching could help. BER is not a disease, but an abiotic condition. If the rest of the tomato isn't rotten, you can just cut off the bad part. It's not something that spreads, so if you get the watering problem straightened out, the tomatoes that are formed later will be okay.

Where I live, calcium is rarely the problem, but if you live in an area where calcium is lacking in the soil, you can find products at the nursery that address that problem. Egg shells take too long to decompose and won't help your current plants.

Blossom end rot