Something must be overwriting the layer height with some kind of compensation. Take a look at the g-code and make sure the code's Z increments are what you expect. If you have say 0.2mm as a layer height, and the g-code shows 0.3mm between layers, then the problem is Cura. However, if the layer height is 0.2, Cura g-code confirms this by each layer incrementing by only 0.2, but the printer is physically moving an additional 33%, then I'd say you have a calibration issue with the steppers and the belt gear. What is odd is that it worked before. I've seen the wrong belt pulley installed before that would cause this, but in this case, yes, I'd say a bit slipped in the firmware somewhere. You need to re-calibrate the z-motor's gear circumference... wherever you do that on your machine. You could just try reloading the firmware. I don't say that lightly considering the usual risk. Firmware is indeed where this kind of calibration lives; just make sure it is not something that got bumped in Cura first.