Sorry, I should have said it's on the bottom of the tomatoes.

Opposite of stem right?

Yes. Could it be blossom end rot?

I think so.

Gah, I saw your first post when it said something else ("top rot"). Sorry for confusion.

I realized it was opposite the stem before i saw your second post...lol...sorry

BER, is one of the prevalent container growing problems.
Although fundamentally it is due to calcium deficiency ( in the fruit) , bu it is widely believed that the problem is due inconsistent moisture level in the soil. The smaller the container, the higher probability of it happening. Combined with that could be some varieties that are prone to BER.

The good news is that, once the moisture level is kept consistent, BER should not happen. JMO

Definitely looks like Blossom End Rot.