And I did just discover aphids on one of the leaves this afternoon. I now suspect they transmitted something to the tomato plant but as a total newbie I cannot verify this and was hoping someone else has experience with this issue.

Doubt it's a disease. More likely environmental.
Have you fertilized? If not, please do. If yes, please explain what & when applied.

I haven't fertilized yet but plan on doing so this week. I'm a vegan/organic grower so I just plan on just adding whatever basic mix I can find. The compost I used may have been lacking in a nutrient or two. I have added a small amount of compost around the base last week and it's been watered with that twice. I will probably be adding more soon after some fertilizer.

The growth on the plant has severely slowed down and it's no longer having flowers open up. It's just holding onto the 15 or so big tomatoes on the plant and sitting there. Some of my melon plants are doing the same thing, which leads me to believe I need to add more nutrients to the soil. Thanks!