Question

The Talmud in Shevuot 8b brings Rabbi Shimon’s reasoning regarding the sin of a woman giving birth (that she jumps up and swears) not merely as aggadic literature, but as an actual sin—to the point that it falls under the definition of “sin” for purposes of the atonement of the goat sacrifice.
What kind of sin is this, if it is not even certain that she did it at all (yet every woman who gives birth brings it)? And even if she did, why is there a sin offering for speech (and perhaps even for thought)? And finally, if it is clear that she will jump up and swear, then this is complete coercion, isn’t it?