Does your Contributor Level drop when not actively sharing games ?

No.

Your CV changes, if the price on Steam of the games you contributed changes.

No, it drops when games get marked bundled or free or their price on steam drops.

Some games have been bundled and not yet marked so when you make your gift, so it looks like it's full CV, but after it's marked as such, it drops down.

If your gift was before it had been bundled, the CV obtained won't drop.

But as Oppenh4imer has mentioned, it's related to the price of the game on Steam (base price, not sale price)

If a game was gifted when it was $50 and it had Full CV = 50, or Bundled = 7.5, then the price on Steam drops down to $20, then your CV would drop down to 20, or 3, respectively for that game.

A great example is Ori and the Blind Forest, which was until recently, $20 base price.