The Mega Challenge lasts 1 Week from Tuesday to Tuesday so if I understood correctly you didn't come for 2 Days or 30 Maximum Round that you didn't Play.
So it seems quite normal to me that you are Lost 3000 Minimum Place. Because the Mega Challenge and a grouping of all the available Challenges with almost all the Free Games.
How and how is the ranking calculated?
You win medals by playing the Free Game according to your ranking in each Free Game you earn more or less, so the more you add up and the more you have the more you are at the top of the table.
Hoping to have been able to help you a little bit.
Hello 20th at the mega challenge it's only 10000mdpts and moreover your final position is determined only at the end of the game at 23h59, so you should know that some players participate in the games late in the evening. You can lose a few places on your weekly challenge and therefore back off sharply on the mega challenge.
On the last day of the Previous Mega-Contest, the bubble on the left of the screen indicated to me that I was 20th. After clicking on it I account that this place corresponds to 20,000 m+ if my memories are good. I haven't been able to connect since Sunday, but I came today and oh surprise: no more m+! Is the bubble a lie, or did 3000 people play on Sunday afternoon and I got kicked out of the ranking?
Now the bubble tells me that I'm 5th, I don't really know how the ranking is done, but I'm recently registered, I haven't really understood the How it Works this thing yet, so if someone had the kindness to help me, it would be cool!