Hello @Kay_Wil
This looks to me like a mesh resolution issue, and that your polygons are getting stretched in that area. When this happens you must either subdivide your mesh further, or retopologize it to quickly resurface and relax the polygons back into shape. You can also use Sculptris Pro mode to update your mesh topology in real time to prevent polygon stretching.
You can use Dynamesh or ZRemesher to retopologize your mesh, which will redistribute your polygons and reshape them to be as close to square shaped as possible. This is the ideal sculpting topology. Polygons that are overly stretched or oddly shaped will start to make that sort of distortion in the surface, as well as performing poorly for painting.
Note that Dynamesh will maintain your surface detail up to a point, but is most effective at low to medium levels of mesh detail. After a certain point you will need to subdivide your mesh and project the detail from the original, in order to remesh it while preserving detail.
ZRemesher will also resurface your mesh, but tends to be used to simplify or reduce polycounts. When you eliminate polygons, you will also lose detail, and so ZRemesher is often used with subdivision levels and the various methods for detail projection to restore the detail.
Good luck! 
