Could you perhaps give us a botanical name? Am guessing no one knows what it is; I don't know that name.

Bamboo, do you have a Hanson's Cherry Bush? If so, they're not the easiest plants growing indoors..
The Hanson's Cherry start blooming 'white flowers' around May..fruit are dark purple to black and edible.
They prefer living outdoors, especially in the ground, but also sold as houseplants.
They're used grown as bonsais, which is what led me to believe you're talking about Hansons.

Did you prune/trim its roots before placing in a bonsai pot? If so, your plant is bound to drop leaves. It's now a matter of adapting to its new environment.
If you trimmed roots, did you prune stems, too? Bonsai's need balance. I believe, 1/3rd roots to 1/3rd foliage.

For the time, allow bright light, although Hansens need full sun to develop fruit. But until your plant acclimates, place in bright, indirect sun.
Place in a saucer with stones/pebbles. Shower the entire plant, soil and foliage. 'morning preferably'

Are its leaves yellowing or browning before dropping? Toni

This plant is probably the very vigorous Eugenia myrtifolia 'compacta' (aka Syzgium paniculatum, and commonly known as dwarf brush cherry). Even if it was Prunus besseyi (bush cherry), there is no reason that when a proper spring repotting and root pruning is performed that the plant would be 'bound' to shed foliage. Look to other cultural issues like watering (too much/too little), over-fertilizing, low humidity, sudden decrease in light levels, or poor timing of the repot (it's difficult to tell if timing is appropriate because you don't indicate where you live - state/USDA zone - in your user info).

If you only acquired the plant 3 weeks ago, it seems like it would have been prudent to let the plant get settled and acclimated to its new surroundings before you undertook a repot. Odds favor the likely cause of the plant shedding foliage as being sudden change (decrease) in light intensity between its former home and its new.

Bonsai need balance? 1/3 roots to 1/3 foliage? What does this mean?

This plant should not be misted, though humidity in the 40-60% range is preferred.

Al