For those whose pastors are telling them that they have to stand after receiving Communion, refer them to this web page at the US Bishops’ Conference that contains a response on the matter from the Vatican. This is a definitive answer.
Dubium: In many places, the faithful are accustomed to kneeling or sitting in personal prayer upon returning to their places after having individually received Holy Communion during Mass. Is it the intention of the Missale Romanum, editio typica tertia, to forbid this practice?
Responsum: Negative, et ad mentem. The mens is that the prescription of the Institutio Generalis Missalis Romani, no. 43, is intended, on the one hand, to ensure within broad limits a certain uniformity of posture within the congregation for the various parts of the celebration of Holy Mass, and on the other, to not regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free.
In the implementation of the General Instruction of the Roman Missal, therefore, posture should not be regulated so rigidly as to forbid individual communicants from kneeling or sitting when returning from having received Holy Communion.
There you have it. The GIRM does not and should not be used to tell people they can’t kneel after Communion.