We think of tomatoes as summer vegetables, but they can struggle in hot weather.  Summer heat causes problems with ripening, fruit set, and flavor. Most tomatoes grow and produce best when daytime temperatures range from 70 to 80 degrees F, and night temperatures fall between 60 to 70 degrees F. Most of our summer temperatures cause plants to shed their blossoms without setting fruit. 

Hot Temperatures Can Mean Fewer Tomatoes | N.C. Cooperative Extension (ncsu.edu)

Your tomato plants may be suffering from physiological leaf curl.  Heat stress can cause tomato leaves to curl.  If the leaves are not deformed, then the leaf curl is probably a reaction to the hot weather and drought.  High nitrogen fertilizers can also cause leaf curl, so make sure your fertilizer does not have too much nitrogen.

Here is some information on the causes of leaf curl in tomatoes.

E-626-What-Makes-Tomato-Leaves-Twist-or-Curl.pdf (tamu.edu)

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