For some reason, your tomatoes can't use the available calcium in the soil. 1st cut off those tomatoes and throw them out. Try giving them a drink of magnesium sulfate aka epsom salt. It's very inexpensive and you can pick up a box at the supermarket or pharmacy. Add a couple table spoons of epsom salt to a gallon of water, give to your tomato plants to drink drenching the leaves as well. Magnesium will aid in sufficient calcium uptake. There are also foliage sprays you can get from the garden center that deliver calcium/magnesium right to the leaves.

Also, commonly, the first few tomatoes often get blossom end rot & the plant will naturally correct itself as it matures a little bit. This happened to me with all my tomato plants. The first few tomatoes all had blossom end rot. I got rid of those, and all the subsequent tomatoes are just fine & delicious. Last night I made grilled cheese sandwiches with ham & tomatoes, not just any tomatoes, MY tomatoes! They were heavenly!