Q. What should one do after receiving the Holy Communion. Should one sit or kneel in prayer?
A. We take our direction on all liturgical actions from what is called the General Instruction of the Roman Missal. #43 of that Instruction states: “… if appropriate, they may sit or kneel during the period of sacred silence after Communion.”
This, of course, does not answer your question directly. What it means is that it is optional to sit or kneel at that time. Therefore, it is more up to the established local custom, or the custom of the national bishop’s conference, to make the normative determination for each location. Therefore, the best thing for you to do is to follow the custom of the local church under the direction of the priest.
With that said, it seems most desirable to kneel at that time. Holy Communion is a sacred encounter with the Living God and, as a sign of your interior prayer, it is most desireable to enter into the most reverent posture possible. Some religious orders even lay prostrate at that moment.
However, even though it is very legitimate to argue that kneeling is the best practice, and even though this is the most common gesture in most territories, it is good to act in uniformity with the local church and to follow the local custom so as not to draw attention to yourself and to your personal prayer.
Most importantly, at that moment you should strive to tune everything else out and enter into deep prayer with God. If you do that, you can be assured that your Communion will be fruitful.