this sounds like too little water. It prefers water retentitive soil and good light but not strong direct sun. partial shade being the best. it also might be too cold if it is outside.

when it warms up prune it back by half. It is often sold as a bonsai plant.

Yes it does get sold as a bonsai. However as a bonsai it should be grown indoors. But it is susceptable to draughts, overwatering, underwatering, overfeeding, moving to a different position (often shows this by leaf drop), too much light, not enough light and last but not least - dry air.
Welcome to the wonderful world of bonsai.
If you bought this as a bonsai from a shop, it will need to be repotted into a fresh potting medium as it will probably be potted up in clay (mud) which does not allow the tree roots to breathe and is difficult to re-wet when it dries out. Most survive six to twenty four months before failing.
New bonsai enthusiasts would be better off with a different type of tree/shrub to grow such as Cotoneaster horizontalis (Rockspray cotoneaster) because they are sooo easy to grow.
Serissa foetida (Tree of a thousand stars) is a difficult tree to keep alive for newbies. I wish you well and hope you prove me wrong.