The Shulchan Aruch (190:1) cites the Terumas HaDeshen that if a woman were to have a hargasha (a sensations that would indicate bleeding - see pischei teshuva for 3 types), she must assume that she bled even though she didn't notice any blood. The rationale is that the Hargasha itself is evidence that there was actual bleeding.
Rav Zeira asks whether a woman who is not involved in taharos should examine herself prior to relations before being with her husband. R. Abba asks whether she should examine herself after having relations to figure out whether they need to bring a korban. The gemara responds to both questions by saying that she shouldn't examine herself because if she would, he would be hesitant to have relations with her. Rashi (d.h. im kein) seems to interpret the gemara to be questioning whether chazal impose such an examination on her. According to this approach the response of the gemara is that by Chazal imposing a requirement to do a bedika, they are indicating that there is a concern that she will bleed, this will cause him to abstain from having relations with her. However, if she were to decide to do a bedika on her own, we wouldn't stop her. Tosafos on the other hand seems to indicate that by her doing a bedika, even as a chumra, would indicate that she must have had a hargasha and cause him to abstain from relations with her. Why would he abstain from relations? Let her do a bedika and if it's clean it would indicate that she is not a Nidah. It seems that Tosafos supports the opinion of the Terumas HaDeshen and if she were to have a hargasha, even if followed by a clean bedika, she would be considered tamei. Therefore we are afraid that he will abstain from relations even if the bedika is clean out of fear that she had a hargasha.
