It's certainly not illegal to arrest a nursing mother if police have a warrant to do so. What you don't mention is why you were arrested, but assuming there was a warrant, whether you were breadfeeding or not, the police can arrest you when they find you. And Similac is ok. Hospitals feed it to children all the time, without negative effect in most cases.
Responses are for general information purposes only, and are based on the extremely limited facts given. A consultation with an attorney experienced in the area of law(s) indicated in the question is highly recommended. Information and advice given here should not be relied upon for any final action or decision, as the information is limited by its nature to the question asked and the fact(s) presented in that question. THIS RESPONSE DOES NOT CREATE AN ATTORNEY/CLIENT RELATIONSHIP, particularly considering that the names of the parties are unknown.
Years licensed, work experience, educationLegal community recognition
Peer endorsements, associations, awardsLegal thought leadership
Publications, speaking engagementsDiscipline