Week 15 and currently in Japan. Feeling slight fever, dry throat, heaty Flight back is tomorrow.

You poor thing, that's terrible timing with the flight tomorrow. I'd honestly suggest getting to a clinic or pharmacy in Japan today to get checked out and grab some medicine, because flying with a fever can be rough on your body and the cabin air will make your throat worse. In the meantime, stay hydrated, rest as much as you can, and try to keep cool, but don't wait it out hoping it gets better on its own. Better to be safe since you're pregnant and get proper medical advice from someone there who can assess you properly.

since you cant take any random medication, I'd go for natural options like honey and loads of water + isotonic drinks. Take care!!!