I recently had a similar problem on the Y axis of my Anet A6 that I still use sometimes.
Problem was the cheap belt included in the kit.
It had stretched... maybe because of over tightening and the heat of the heatbed above.
And I think that happened pretty suddenly within a short time span and not just over time...
Well, and I had an other problem where I dripped hot glue on the pulley of the X axis and didn't notice it... it was just a tiny amount and barely visible, but was enough to increase the effective diameter (unevenly).
Like @t3p3tony, I would also suggest to test by moving the axis and to measure the actual moved distance, maybe also in e.g. 10mm steps to see if the error is conststent or not.