How come my post count is decreased?

If some of the questions that you responded to were removed by the site administrators, that would delete your posts, so you would have a decrease. I've heard of this happening to others, though I haven't experienced it personally.

i guess this happens when you do something illegal, or if you have unknowingly flouted some mylot posting rules. might be a good idea to go back to read the rules and see whether there's something you've been doing wrong. I have come to a point when i seldom check my post count. so I won't really know if a couple are missing. call it trust in mylot i guess!

The reason why your posts decreased may be because myLot periodically deletes useless messages, I've lost about 50 messages here too due to that cleaning but it's normal as the community is growing they have to maintain the discussions.

No, that has never happened to me but I wouldn't doubt it if it's so.