If one is in doubt about the state of his ablution, he does not need to perform a new ablution.
If one is certain that he has nullified his ablution and doubts whether he has purified himself or not, he must perform a new ablution.
Find out what invalidates ablution
- Abbad ibn Tameem related that his uncle queried the Prophet  about a person who feels something in his abdomen while praying. The Prophet  replied, "He should not leave (the prayer) until he hears it or smells it"
[Muslim, Abu Dawud and at-Tirmidhi]
- Says Ibn al-Mubarak, "If one uncertain about his condition of purity, he does not need to perform a new ablution."