Hi, Kelly —
To add to what Mike has said, it is possible to do a baptism or a confirmation in an invalid manner, for example, if a baptism is done in the name of the Creator, the Redeemer, and the Sanctifier. Such a baptism would not be valid, and a tribunal could declare that it is null and must be repeated. Even ordinations can be declared null. But this is not the case you speak of; you're talking, presumably, about someone who after baptism ceases to believe Catholic beliefs. I do not know whether an adult convert who is baptized under false pretenses could have his baptism declared null.
The thing about marriages is that they depend on the consent of the parties at the time of the marriage.
Comparing cases, just as nothing that happens after the marriage affects whether it is null or not (although it might shed light on what happened before the marriage), so nothing that happens after baptism can affect whether it is null.