Even just visiting Japan (I'm originally from Japan) causes this for me. Short stay is fine, but I feel really foreign, maybe more so than "foreigners" as people in there can detect disconnect of my appearance and mindset by my subtle deviation from expectations of how I act in there.
I have been to Japan twice for 9 months in total (originally from Germany). I didn't experience culture shock in Japan at all, but I also never really came back. Reverse culture shock can last for long. It is impossible to talk about this experience to anyone but those who have experienced Japan (or Asia) themselves for long time because most people can not even imagine how different cultures can be.
I don't know if this played any role in your experience but that seems to be what I have been hearing around me...