"The police pulled me over" means the police thinks I did something wrong (for example, was speeding) "The police flagged me down" just means they wanted me to stop but NOT necessarily because they thought I did something wrong. For example, if the police were looking for information about a possible criminal who had been in a particular area, they might flag down cars and ask them if they had seen a man running in the street.

Is my guess correct? No. They mean slightly different things. 1. The police pulled me over. This refers to the instruction to drive to the side of the road. 'Pull over' can be both intransitive - "I felt unwell so I pulled over (to the side of the road)" - or transitive: "The police pulled me over". 2. The police flagged me down. This refers to the signal which the police officer made, telling me to stop. It doesn't necessarily involve pulling over to the side of the road. The police might simply tell you to stop where you are - in a middle lane, for example.

Eliya, thank you for your answer!

They both mean the same thing, but "pulled me over" is more of a certainty that the police took you to the side of the road, whereas "flagged me down" could be "they flagged me down in the middle of the highway" it might be because they wanted to pull you over, or for other reasons.