According to Tosphot in Yevamot page 93 and the Rashba in Kidushin page 66 a claim of I am sure is believed even against a status quo. But this is subject to argument. There is a Tosphot in ketuboth page 26 where Tosphot says that raban Gamliel held the woman is believed when she claims I am sure in a certain case only when a status quo supports her. There is also a case in which Tosphot holds “I am sure” only helps a claim when there is no status quo against it, or when there are two statuses, each one of which is cancelled by the other. An example of the last case is Ketuboth page 22 where two witnesses come from abroad and says a woman's husband had died, and thus, she can remarry. But two others come and say her husband did not die. If she married one of the witnesses that said her husband died, and she herself says she is sure, then she can stay married. In this case there are two status quos that cancel out each other. One is he status as a married woman. The other is the status that a woman always checks before she gets married.-----------------------------------------According to תוספות in יבמות page צ''ג and the רשב''א in קידושין page 66 a claim of "I am sure" is believed even against a חזקה. But this is subject to argument. There is a תוספות in כתובות page כ''ו where תוספות says that רבן גמליאל held the woman is believed when she claims I am sure in a certain case only when a חזקה supports her. There is also a case in which תוספות holds “I am sure” only helps a claim when there is no חזקה against it, or when there are two statuses, each one of which is cancelled by the other. An example of the last case is כתובות page 22 where two witnesses come from abroad and says a woman's husband had died, and thus, she can remarry. But two others come and say her husband did not die. If she married one of the witnesses that said her husband died, and she herself says she is sure, then she can stay married. In this case there are two status quos that cancel out each other. One is he status as a married woman. The other is the status that a woman always checks before she gets married.