AdMob terrible fill rate on growth

Does admob preallocate ads amount for account based on previous days?
When I have 20-70x more requests than day before (for example after being featured in Mexico) my fill rate drops from 90% to 1%, but sometimes it can handle it day after and sometimes not.
Is it preallocation as I said or request DoS (I don’t see reason for this one, I’d bomb clicks instead of requests) or something else?
I don’t see this problem in tier 1 english countries like USA, Australia, but I see in Spain.