Tomato plant unhappy, why?

Tomato plants – all doing well but one!
Question Details: I have five tomato plants and all of them are doing really well except one—they were all growing well and vigorously, and all seemed healthy. When they got to about four feet tall, they started producing flowers and then green fruit. About that time, say, last week, one of them started to look sun-stressed and wilty. Then it started turning yellow. All the others look great, and the sick one is right in the middle. It’s been at least a week and it’s still going downhill. I can’t imagine it could be over or underwatered, since the others are fine. Any ideas?