Rabbotai,
The Gemara tells us that R. Chiya's wife had a lot of pain in childbirth since she would always give birth to twins. She told R. Chiya that her mother had told her that her father had accepted kiddushin for her when she was young so her marriage to R. Chiya wasn't valid. This report, if accepted, would make R. Chiya's children mamzerim. R. Chiya said that we don't have to accept that report even if there may be witnesses elsewhere whom we could question.
Even if we accept his logic and consider her story invalid why don't we say shavya anafhsei chatiche d'issura? Her report would make her ineligible to stay with R. Chiya since she seems to believe it herself. If she says that she's assura to him she shouldn't be allowed to have relations with him. Might it be because it's obvious that she's using this as an excuse to avoid childbirth pain?
B'kavod,
Sam Kosofsky
Good question and good answer. Firstly, she does not claim to know it is true, just that her mother said so. Secondly, the Gemara clearly implies that she was trying to get out of the marriage (or at least relations) because of her pain. The Meiri here therefore explicitly concludes that whenever we have reason to believe that a woman is saying she is forbidden to her husband just to get out of the marriage, we do not say Shavyei Anafasha.
All the best,
Yaakov Montrose