Your body IS being constantly pressurized by the atmosphere, and your various sphincters are used to that. Presuming the suit doesn’t pressurize your body beyond what it’s used to (at which point breathing would be difficult), there should be no unexpected anal excretions due to the suit.
But the pressure from the atmosphere applies to both sides of the sphincter, resulting in zero net pressure. Unless the suit actually does press against the outside of the sphincter like it does the rest of the body, I think OP’s concern about the suit squeezing you like a tube of toothpaste is valid.
Maybe the suit only applies a few PSI instead of the full 14.7, which it seems like one’s sphincters would be able to withstand.
I think it’s like a third of an atmosphere or something. Enough to comfortably achieve the same partial pressure of oxygen as normal Earth air, by providing it pure.