yes as he says "==" is testing, "=" is setting values:
if(something == something)//testing
value = value2; //setting value equal to value 2
if(digitalRead(xStopMin) == HIGH){
xIs0 = true;
xPos = 0;
Serial.println("xMin");
}
you say xPos = 0;, if you accidentally pressed the button,. it may reset the value, but i dont hink this is your issue.
put _Serial.println()_s around where the xPos and yPos value would change and see if they make sense.