I had this problem also years back. Irregular watering maybe the cause.
Jun 30, 2011 Rating
Sounds like Blossom end rot... by: Rick from CGFF
Hi Erika,
I'm wondering if this could be a classic case of 'Blossom end rot'.
It can be brought under control by adding calcium to the soil and feed, and by maintaining correct levels of moisture.
Tomatoes that are planted early, especially in cold soil are more likely to develop the condition on the first tomatoes. The severity may subside on later fruits if you can get the calcium issue addressed.
Water them enough to maintain a steady even growth. Mulching can also be employed to help maintain the levels of moisture in the soil.