You have to perform Istinja' (cleansing the private parts with water after urination or defecation) after relieving yourself, especially the urine discharge area after the urine completely stops, then make Wudu'. If
you make Wudu' and then doubt whether something has come out from you or not, the general rule is purity (as long as there is no proof to prove otherwise) and there is no effect for the doubt. The Prophet (peace be upon him) said:
A person should not leave (Salah to renew his Wudu') until he hears a sound or finds a smell.
The basic rule states: Certainty cannot be annulled by doubt. It is permissible to perform `Isha' with the Wudu' made for Maghrib but if you renew Wudu', it will be better.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.