Disease, BER or Pest? I really can't tell

[TNTiger]

Attached are several pictures of a tomato with some sort of internal rot. I have no idea what this is and whether there is something I should be doing about it (other than getting rid of the tomato itself!).

The outside of the tomato looked fine until the day (or so) before picked. I've seen several like this now from at least 2 different plants. I'm not seeing any evidence of insects on these and the plant seem to be doing ok in general. Fortunately, I'm getting far more good tomatoes than rotted ones like this but I'm seeing enough to be concerned.

Any ideas what I've got here? Thanks!!