In the back wall of the inside of my vagina i dont find any lumps but there's first a thin wall where i can feel stool and above i feel a thicker am tougher wall opposite the cervix. is that normal?

Probably normal: a finger inserted deep into the vagina may feel the gut wall posteriorly (toward back) which may have stool in it. Anteriorly, toward front, one may feel the urinary bladder wall which may have urine in it. It may feel thick because the finger will feel the vaginal wall, the bladder wall and perhaps the inside wall of the abdomen. If any question, have gyn exam.

Yes that is normal: The back of the Vagina borders the anus and you should be able to feel stool if you push hard enough. The opposite wall borders the muscular bladder which is also normal. You are ok.