When you mix curd with hot rice and eat, digestion problems are bound to occur.Generally, when you mix cold food items with hot rice, it affects the property of the rice and leads to digestion problems.
This may lead to successive stomach disorders. So, it is always advisable to let the rice cool a bit and then eat. It is also not advisable to mix hot rice with cold curd under the impression that it will be consumed only during lunch hours which may be 3 hours later.
If it is an emergency situation when you must mix and take, then first mix a small quantity of cold milk with the hot rice and then add curd and mix. With this, the curd rice also will remain fresh till lunch time.
It is also not advisable not to mix cold buttermilk with hot rice.