Legal Question in Criminal Law in California

Probably, unless there are some unusual additional details you haven't provided.

Law enforcement officers who unexpectedly see evidence of a crime don't have to ignore it. They also don't have to act on it immediately. Their decision not to arrest you on the spot does not mean you were off the hook. Prosecutors remained free to charge you until the statute of limitations had run out. (How much time they had will depend on the charges, but a 30-day delay was surely allowed.)

There are limits on when officers can actually arrest someone for a misdemeanor. That's something they might not have been allowed to do 30 days later. But your question says you were arrested for something that happened afterwards, and that you were charged both for that offense and for the earlier narcotics violation. That sounds perfectly legal to me.

Please note, though, that I might see things differently if I had more information. Users on this site need to be brief, and the details they leave out sometimes prove to be significant. You should get a lawyer to defend you (the court will appoint one if you can't afford private counsel); she will be able to give you better guidance after you tell her more about the facts.

Good luck.