That's normal, once the temperatures get hot. Tomato flowers refuse to set fruit. Where are you? Seems kinda early for that to be happening, even in the warmer parts of the U.S.

Temps are the primary cause but there are other contributing factors too. See the FAQ here on Blossom Drop.

http://faq.gardenweb.com/discussions/2766768/why-are-the-blooms-on-my-tomato-plant-dying-and-falling-off

That's interesting about overfertilization with nitrogen, Dave. I didn't know that.