OK so that is what happens to block 18290. It looks like you were mining only first 3 minutes of that block. So this is normal. But I don't get it why you got double from block 18291 since you didn't increase hashes... Did you change anything? The only thing I can think of that could be is a difficulty error in a score system and you change it from 2 to 1. But I guess someone would report that already... Looks really strange...
I haven't changed anything. so it's kind of confusing. Also, I'm on difficulty 6
Losing most of my reward for a short disconnect seems to be very unfair in my opinion. I would really appreciate if slush would explain what happened.
This is the way the scoring method works in order to prevent pool hopping. As the round goes on, shares from earlier in the round have less and less value until they are worth absolutely nothing, so if you only work for the beginning of the round and then disconnect, it is possible to get 0 reward even though you have submitted shares.
Read up on the "Slush method" of scoring for more detail