Not sure this is the appropriate sub-forum for this thread...
In any case, I've spent multiple years overseas speaking various languages and although I have felt 'foreign language fatigue', it was different to how you're feeling. I suspect what you're feeling is probably something more so related to mental or physical fatigue. I'm not a doctor so I can't really say for certain. Maybe it's a combination of jet-lag and a bug?
Foreign language fatigue, or whatever the actual term might be, for me is why I feel mentally drained and my mouth feels extremely exhausted. I remember going to parties in my first few months while studying in Tokyo and halfway through I'd become incredibly quiet to the point that I couldn't speak. Both my mouth and mind were exhausted. My mouth muscles weren't used to making the movements necessary for Japanese speech/sounds and my mind was working over-time trying to translate, conjugate, remember and construct sentences every single second. I remember every time this would happen my friends would become concerned and ask me if everything was OK. They'd think that I was having a horrible time or that something bad had happened. I didn't even realise it myself until it became a recurring pattern.
As an aside, what country are you in and what foreign language is it? Certain languages like French that utilise different mouth muscles for speech are non-existent when speaking English making it all the more exhausting to speak it for prolonged periods of time.