Must police tell me why I'm being arrested?

Yes. Police must inform you of the grounds of arrest immediately. This is a constitutional right.

Under Article 22(1) of the Constitution, every person who is arrested has the right to be informed of the grounds of arrest as soon as possible. BNSS Section 47 also mandates that the police officer making the arrest must communicate the full particulars of the offence for which the person is being arrested. Failure to do so makes the arrest illegal. If you are not told the reason for your arrest, you can challenge the legality of the arrest before a Magistrate or file a habeas corpus petition in the High Court. This is one of the most fundamental safeguards against arbitrary detention.

• Article 22(1) — Right to be informed of grounds of arrest
• BNSS Section 36 — Procedure of arrest