Problem with stl exporte file

I made a lamp shade uding a box, makeing it holow 0.6 mm wall thickness. Made The corner rotund. I exported it to stl. I sliced with S3D. Everything ok. Started to print. 3 4 layer ok than The wall thickness doubles altdough în The 3d model and slice idont see the thickness dubleing. When i tryied The 25mm calibration cube from thingivers it printed fine. So i think when its exporting The stl file its doing some damage to The design.