It's most likely a nutrient thing, deficiency due to demands of heavy fruiting. But just to cover the bases:
- How are you fertilizing?
- Are you growing in-ground or containers?
- Where are you located and how has your weather been?
-Are there any other unusual symptoms on the plant?

I use calmag plus with most waterings, and I have been using miracle gro nature's care vegetable, fruit, and flower concentrate that provides only nitrogen. I also applied some shake and feed tomato food a bit ago. When I planted it a bit after mother's day, I added some bone meal and compost to the soil. It's growing in the ground in Maryland, where it has been pretty hot (90s) for the past two weeks. Since it hasn't been wilting at all, I haven't been watering too much in the past two weeks, though.

Once they start fruiting, tomatoes need more phosphorus and potassium than nitrogen.

Thanks. I just watered it with some Schultz 10-54-10. I'll see how it goes. Interestingly enough, and I know they're different plants, but the same regimen is making my Caribbean red habaneros set fruit extremely prolifically.