My level just fell back! What's wrong??

Refer to the FAQ. It helps you understand your situation.

Most likely a game you gave away decreased in value, probably due to being bundled

Search the forum. There are literally countless threads in which people explain the reasons it may happen. There is a search function on the left side in the Discussions page.

Some of the games you gave away were probably added to the bundle list. A game's cv decreases to 15% of the original value when it is added to the list.

See this.

Main ways your level drops:

1. A game was bundled prior to your giving it away, but the bundler hadn't gotten around to adding it to the bundle list until now. [If you give a game away while the game is unbundled, you'll always keep unbundled credit for it, even if it goes bundled later- however, sometimes a game won't properly show its bundle status on SG for a while, causing the appearance of retroactive bundling.]

2. A game you gave away has dropped in retail price- this will lower your contribution accordingly, as value is always based off the current retail price of a game, as Steam reports it.

3. Steam is bugging. It happens. It usually resolves quickly.

4. Games you gave away were artificially inflated in value at the time you gave them away, due to Valve's poor design in handling sales. Essentially, if a $20 base game doesn't go on sale, but the $30 Deluxe version that shares the same store page goes on sale cheaper than $20, the $20 base game will also report as the Deluxe game. For example, if the Deluxe game goes on sale for $12, the base game will also report as having a value of $12/$30. Thus, SG reads that $20 game as having a retail of $30, as SG pulls price listings directly off Steam.
   Once the sale ends, the SG value drops accordingly, thus also creating an illusion of loss.

As you go higher in levels, losses start having lower and lower chances of dropping you a level, given how little individual giveaways start meaning against the whole of your contributions.