A new update to CS:GO was published last night by Valve.
With the recalibration of the competitive skill groups, all players will be affected by the changes. On Valve's blog for Counter-Strike, the game developer explains the situation.
Competitive Skill Groups are undergoing some recalibration.
Typically when we ship changes to CS:GO’s matchmaking system, the adjustments are small enough that we don’t include them in our release notes. However, today’s update affects all CS:GO players so it requires some explanation.
When you launch CS:GO, you’ll notice that your Skill Group is not displayed–you’ll have to win one more match to reveal your Skill Group. Most of you will notice a change to your Skill Group, but some of you may find that you were already in the right place.
So jump into a match, or two, or however many it takes for your next win!
Judging from the reception on Twitter, most players seem to have received higher ranks after their first win after the update.