Your artifact is at full power

I'm a priest. I'm level 110, I completed a raid and then got a quest to go to Orgrimmar for some important business, watched a cut scene where the goblin leader showed a glowing thing to the warchief, then I went to silithus and did all the quests I could find using shadow spec with level 75 on my artifact, then I went to a random battleground, after that switched to discipline spec to spend the points and got the message

"Your artifact is at full power"

but my artifact is not at full power, it's at 73, so what can i do?

It says your artifact is at full power because Blizzard recently made a change that gives everyone 127 ranks in their artifact, ramping up over the next couple of weeks until the Battle for Azeroth prepatch is available.

However, to unlock this for a class, you need to have done a quest with Magni onboard an air ship of your faction in Silithus. To get this quest, in addition to the rest of the Silithus quests, you also need to have done the quest with Magni in Sholazar Basin. If you haven't done this quest yet, go do it now, then log out and back in. A new quest should appear to empower your artifact, giving you 127 ranks in each of your artifacts. You might have to actually open your artifact trait window by switching specs and shift-right clicking.

After you have unlocked this quest, artifact power no longer is usable and you can safely vendor all artifact power tokens you receive. Your artifact will gain new levels on its own, up to max Concordance rank (which should be 100).