The feeling of the worshipper that something has come out from his rectum or his penis does not invalidate his ablution, and he should pay no attention to it, because it is authentically reported from the Prophet (Sallallaahu alayhi wasallam) that he was asked about something like this, and he said:
áÇó íóäúÕóÑöÝú ÍóÊøóì íóÓúãóÚó ÕóæúÊÇð Ãóæú íóÌöÏó ÑöíúÍÇð
"He should not leave (the prayer) unless he hears a sound, or smells an odour..." [Al-Bukhari no. 137 and Muslim no. 361]
However, if the worshipper is positive that an odour, or urine or the like has been emitted, then his prayer will be invalid, due to the corruption of his state of purity, and he must renew his ablution and pray again.