A bonsai would repeat season wise exactly what every tree of that species would do at a given time each year or during a given season.It is important to get to know all the habits of that family & species,inappropriate watering,insufficient light or warmth,overgrown root system resulting in a pot bound plant,improper feeding,could all lead to yellowing of leaves.Since you have had the plant for a few months only,I presume you bought it from a reputable nursery so many of these problems can be ruled out as they would have been taken care of,if it is a decidous tree ,find out if it is keeping to it;s natural cycle.,were yellowing oreceeds leaf fall .

This site has a lot of advice applicable to your problem.

However, it is very unlikely that the reported problem was caused by a single watering last night. Leaves turning yellow and falling off can be caused by BOTH over-watering and under-watering.

Sounds like you over watered it, it may have been sitting in water all night. Take paper towels and push the soil aside enough to get them down into the pot…you can twist the towels into a cone shape. Turn the pot so it’s elevated in one side so the water drains toward the paper towels and let the towels soak up as much water as possible. Keep changing the towels when saturated. Hopefully, you caught it in time. Good luck.

have tou trimmed the roots. if not then the roots might have grown bigger, making it hard for the pot to support the growing plant