If you're growing them indoors you have to pollinate them yourself. If you don't do that, the flowers simply die and fall off.

To pollinate tomatoes you can shake the branches manually, or (more preferably) get an electric toothbrush and use that to vibrate the flower clusters at the stem. Do that every day or two when the flowers are blooming (the yellow outer petals are curled back) and you should have good fruit set.

The best time to do this is between about 10am and 1pm, or whenever the humidity levels are highest which promotes stickier pollen.

Same thing happening to me, upper leaves are also notlooking healthy, looking small.