You have Blossom end rot...
Control of blossom end rot is dependent upon maintaining adequate supplies of moisture and calcium to the developing fruits. Tomatoes should not be excessively hardened nor too succulent when set in the field. They should be planted in welldrained, adequately aerated soils. Tomatoes planted early in cold soil are likely to develop blossom end rot on the first fruits, with the severity of the disease often subsiding on fruits set later. Thus, planting tomatoes in warmer soils helps to alleviate the problem. Irrigation must be sufficient to maintain a steady even growth rate of the plants. Mulching of the soil is often helpful in maintaining adequate supplies of soil water in times of moisture stress. When cultivation is necessary, it should not be too near the plants nor too deep, so that valuable feeder roots remain uninjured and viable. In home gardens, shading the plants is often helpful when hot, dry winds are blowing, and soil moisture is low. Use of fertilizer low in nitrogen, but high in superphosphate, such as 4-12-4 or 5-20-5, will do much to alleviate the problem of blossom end rot. In emergency situations, foliage can be sprayed with calcium chloride solutions. However, extreme caution must be exercised since calcium chloride can be phytotoxic if applied too frequently or in excessive amounts. Foliar treatment is not a substitute for proper treatment of the soil to maintain adequate supplies of water and calcium.
This is how I grow great maters;
Well drained soil is a must (add Pete moss if you have to the dirt and mixing well)
When planting put powder forum lime in the hole (sold at Lowes for around $3.00 a large bag)
Add the plant and fill with water... let sit until water drains and is only moist
Add more lime, water and cover with dirt
Add organic fertilizer (sold at Lowes also for around $8.00 small bag)
Cover with mulch
Add water as needed or every other day when no rain is around
Hand dust the lime on the foliage of the mater plant to keep bugs away and also feed the plant
I grow maters the size of small pumpkins