My karma decreased, and I no longer show as being active in certain projects

Karma decays over time: what is worth 10 points today will be worth less as time goes by, and this is to reward people who are actively using Launchpad. The karma calculation is discussed in more detail at https://help.launchpad.net/KarmaCalculation

As with karma (and in part because it is calculated based on your current karma) the list of projects you are most active in, present on your Launchpad homepage, will vary over time. If you contribute to a project in the past but at some point stop participating, over time that participation will expire.