I live in the U.S., but I know the flu here has been awful this year. May I suggest you call your doctor office and ask them what the symptoms are of the flu bugs going around? And in case it hadn't reached there from where you visited, do you have anybody in Japan you could contact to ask them if they could do the same with their doctor and get back to you?
The peripheral neuropathy in your fingers you are experiencing does not sound like a typical flu symptom though and I think you should see a doctor about that and they surely would want to check out the chest pain as a huge priority.
Some of the symptoms you describe with the pain in jaw muscles when you yawn and even potentially some soreness in the face sound like you might also have a TMJ issue, so you might want to go to a dentist who has knowledge of TMJ dysfunction or an oral and maxillofacial specialist who does. Do you know if you clench or grind your teeth at night or not?
Hmm insects.. I am not quite sure, but do not think so, otherwise I could have noticed it.
Flu - possible, I was few times in public places.
Did you get any insect or spider bites on your trip? Was there anyone around you that might have had a flu virus?
I had to add that during fever time I also had pain in my eyeballs, - the typical symptom I have with flu.