Problem with my tomatoes can anyone help?

[stardust]

Hello
I have a problem with one of my tomato plants I have 4 in my little green house but there is only one affected does anyone know what it might be Ive uploaded a picture of it.

Thanks

[Tomatoking]

Blossom end rot. Lots of threads on this one, mainly caused by underwatering.

Mr TK

[Flummery]

Or irregular watering too, I think?