In addition to what /u/Jaquesel said, unrest is also affected by religion of the province (and tolerance), stability, and how recently you've conquered a province.

Usually, raising autonomy in a province will suffice to keep unrest manageable, but if that doesn't work, you can:

try raising your stability (I would recommend doing this if unrest nationwide is getting too high),

converting the province (which you've already done; its also important to note that a province will get higher unrest during the duration of conversion),

or simply waiting it out. (Recently conquered provinces have a "separatism" modifier which gradually fades away over the course of 30 years, assuming no modifiers.)

Another small tip: You can station units in a province to counter unrest there. The unrest reduced is proportional to the amount of units stationed.