It could be all kinds of things, you could be tired from the day while the people in, say, India and Australia are just getting up. That seems more likely than the idea that you are better against people of a specific nationality.
What is wrong with me?

You're not learning anything at Game in 5/0.
At least put a 5 second bonus into your "open seeks," and work towards reaching a resolution of the game on the board, instead of on the clock.
Never hurts to study, either. So get with the program, and IMPROVE.
Very Simple.

Take from me - you need to stop playing 5 | 0 games. Go read some of Dan Heisman's material if you don't believe me.
I played blitz like that for years because I thought that was all the time I could spare and I went nowhere. You just can't think and analyze positions long enough to figure out anything in a 5 | 0 game.
If you only have 10 minutes to play, you would be better off just doing tactics trainer or deeply analyzing a random middlegame position.
Good luck!

I went to from 5 / 0 games to 10 / 0 games and my rating increased 200 points.
I think that I am getting too old to analyze positions that fast.
Thank you

Praise the Lord, and Pass the Ammunition.
Good luck playing at slower speeds, @Easeltine.
Add a 5 second bonus (for Game in 10/5) and you'll be surprised how quickly you improve your endgame skills.
P.S., I'm about 300-600 points weaker at bullet than at Game in 10/5. Yes, we are all getting older, unfortunately.

I'm not as good as Laurence Fishburne playing the Grandmaster at Central Park in the movie, "Searching For Bobby Fischer."
I am losing more games at night.
I am on the West Coast and realized that I am not playing U.S. opponents during the evening and I am losing more at that time period.
Could it be that Americans are the ones that I am beating during the day and the winning streaks?