For a few weeks this past month, I was worried that my biking skills were on the decline. Maybe I was loosing weight too fast. Or maybe something was wrong with my bike. Or something like that. Also, one of my hand muscles... coincidentally the one used for shifting, was hurting REAL BAD.
I'd stay in the lower gears unless I went sprinting, because that's what felt natural and I'd take forever to get to work in the morning. And I couldn't maintain the higher speeds on the way home.
Then I went out for Bike Party and noticed my seat was wiggling back and forth. And I noticed that it was also awfully low. So I tightened the seatpost clamp enough so that it would stay still and bodged it roughly into place for the rest of the ride. Then, when I got home, I took a good look at the seat level.
I got all my speed back. Suddenly I felt a LOT faster and more comfortable on the bike. So it's probably been creeping downwards for a while and just kept getting worse and worse.