Rotting tomato fruits

[kendo]

Why are some of my tomato fruits rotting inside

[Flummery]

Is it blossom end rot? Does it start from the bit away from the stem? This is caused by 'erratic watering' it's said. It can be difficult to water consistently in pots and grow bags if you can't keep a constant eye on them, or be there regularly.

[kendo]

This is not blossom end rot it is only visible when you cut the tomato in half