Mon Sotah 28

The Gemara learns Safeik Tumah in Reshus Hayachid is Tamai from Sotah.

Tosfos asks: what's the comparison? The reason we are stringent by Sotah, since there is what to suspect, since there is Kinoi and Stirah. However, Safeik Tumah there is no reason to suspect Tumah more than it remained Tahor.

We even find the gemara in Niddah that asks this as a question, how can we say that someone Toiveled in a Mikavah and afterwords found the Mikvah less than the Shiur (and the question is when did it lose it, before or after the Tevilah) remain Tamia because we compare it to Sotah? By Sotah the Kinoi creates suspicion, but by Tevial there is nothing suspicios?

Tosfos answes: just by Stirah itself (Yichud) doesn't even make it to a Safeik Issur, and we don't forbid a wife after Yichud. The Kinoi creates a suspicion to make the Yichud suspicious enough to say that it's a Safeik. Therefore it's compared to a real Safeik Tumah.

However, in Niddah, there was nothing to create a suspicion (to say it' a Safeik_) when the person Toveled. (That only came afterwords.) Therefore we shouldn't compare it to Sotah where it was suspicious at their seclusion.