That sounds like blosom end rot (BER).

A number of factors are thought to contribute to this problem: planting in poorly drained soil, not enough or too much watering, low soil pH, inadequate calcium in the soil or applying the wrong form of calcium, applying too much nitrogen or using the wrong form of nitrogen, excess levels of potassium, and high soil temperatures.

Usually it is best avoided by applying lime when planting and not over fertilizing the plants. I usually try a bit of lime and keeping the watering constant but haven't had much luck controlling it once it is observed.