Q.Why Are The Leaves On My Tomato Plant Curling Into A Hard Little Ball?
Its either an early girl or a better boy.the lower branches appear to be okay.

Certified GKH Gardening Expert
Unfortunately, your photo did not come through. Identifying the issue will be impossible.
My best guess, given that it doesn't affect the whole crop evenly will be one of several viruses. These cases are not curable, typically.
Here are some articles that will help:
https://www.gardeningknowhow.com/edible/vegetables/tomato/treating-curly-top-virus.htm
https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-leaves-curling.htm
https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-guide.htm