A Muslim could make Wudu and later have doubts whether he performed an act that would nullify his Wudu.
Fortunately, there is a hadith that answers this:
Narrated 'Abbaad ibn Tamim (radi allahu anhu):
This means that the Muslim should not leave the prayer due to a doubt, only unless he hears something, or smells something.
So in this situtation, if the Muslim has doubts whether he performed an act which would nullify his Wudu, and has not heard sound or smelt anything, then he is in fact still in Wudu.
May Allah bless you, and shukran.
1. Sahih al-Bukhari 137, agreed upon