Sorry, I didn't link the photo correctly

Here is a link that might be useful: yellowing leaves

I have the same problem on the bottom and the interior of a large cherry tomato that has been in since March. No spots here, either. Carolyn said it was more likely to be age in the absence of obvious fungus or trauma. Apparently tomatoes lose old leaves and replace them...(I'm rather rookie-ish so that hadn't occurred to me). I did notice when I pulled off some of the worst looking that there were already new leaves sprouting at the node between branch and stem. You might see if this is the case with yours. So far mine is not spreading, nor is the plant suffering any, so I assume Carolyn is correct, as she usually is.