Joffrey, Tommen, and Myrcella would fit your question (before edits): Cersei the queen is their mother, but their biological father is Jaime Lannister and not the king.
And as TLP points out in comments, accusing the wife of cheating on her husband shames her lord husband and is a vary dangerous thing. While the husband cheating on the wife is seen as normal/acceptable in this male-driven society.
Upon further inspection, currently there doesn't appear to be any named bastard offspring via a Lady (non-incest.) (here's a nice list of the known bastards).
As MikeEdenfield states in a comment, Victarion Greyjoy's wife was impregnated by Euron, but Victarion Greyjoy decided to just kill his wife.
As an aside: typically in western society from medieval Europe(a heavy basis of morals for GOT) till quite recently, women who cheated on their husbands and became pregnant were either cast out of the family, or they family would cover up the incident. It would be a very dangerous thing for a noble Lady to be cheating on her husband in the world of Westeros, since we see Men typically hold the majority of the power.