I ate some pizza that had Haram meat. I didn't know it until a friend told me the next day and I saw the bill. What can I do now?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Since you were not aware of the pork contained in the pizza and you knew that only after eating it, you are not blamed for that because you used it unintentionally. This is the agreement of Muslim scholars. The same ruling applies to the one who eats or drinks something Haram without knowing it.
However, if one can vomit it only to keep away from the Haram then it is desirable for the Hadith in Sahih al-Bukhari that, " Abu Bakr had a slave who used to give him some of his earnings. Abu Bakr used to eat from it. One day he brought something and Abu Bakr ate from it. The slave said to him, "Do you know what this is?" Abu Bakr then enquired, "What is it?" The slave said, "Once, in the pre-Islamic period of ignorance I foretold somebody's future though I did not know this knowledge of foretelling but I cheated him, and when he met me, he gave me something for that service, and that is what you have eaten from." Then Abu Bakr put his hand in his mouth and vomited whatever was present in his stomach" .
He did so because he was aware of the fact that using Haram darkens one's heart. We ask Allah to satisfy us with the Halal, not Haram.
Allah knows best.