The universal GIRM actually calls for standing during the Eucharistic Prayer — kneeling is a specific American exception. When the revised GIRM was published after Vatican II, the U.S. bishops asked Rome to allow kneeling to continue, believing American Catholics would be scandalized by the change. Rome granted the exception. So what feels "traditional" to American Catholics is actually a local adaptation, while standing is the older and more universal Christian posture.

The Church's goal is "a certain uniformity of posture within the congregation" — because uniformity expresses unity. That said, Rome explicitly clarified in 2003 that posture norms are "not intended to regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free." Personal piety that leads you to kneel at additional moments (such as after Communion when others stand) is not forbidden — but disrupting the assembly's unity by conspicuous difference goes against the spirit of the GIRM.

The GIRM explicitly accounts for this. Those who cannot kneel "ought to make a profound bow when the Priest genuflects after the Consecration." A deep bow of the body — not just the head — expresses the same reverence. No explanation is needed; the Church has already made the provision. You are not doing something wrong by sitting or standing when kneeling is impossible.

Kneel if you are able — it is the traditional and most fitting posture of adoration after receiving Christ in the Eucharist. The GIRM allows sitting or kneeling at this moment ("if this seems helpful, they may sit or kneel during the period of sacred silence after Communion"). Rome confirmed in 2003 that kneeling after Communion is never forbidden. If your parish custom is to stand until everyone has received, follow that — but once you return to your pew, kneeling is entirely appropriate.

Authority runs: Holy See → Bishops' Conference → Diocesan Bishop → Parish Priest. The GIRM sets the universal framework; the U.S. Bishops' Conference adapts it for America; the diocesan bishop can further adapt for his diocese; the parish priest implements it locally. An individual priest cannot invent new posture norms on his own authority — he implements what the Church has laid down. If something your parish does seems unusual, it may be a diocesan adaptation, or it may simply be a habit that crept in without authority.