Why did the distance requirement for my buddy change?

Solution 1:

It is part of the Halloween event for Pokémon Go. See the official announcement

Starting October 26 and lasting through November 1, you can receive way more Candies than usual. Every Pokémon you catch will award six Candies—twice the normal amount. The Professor will give you two Candies instead of one for every Pokémon you transfer to him. And your Buddy Pokémon will award Candy four times faster than they usually do. Plus, you'll get twice the Candy you'd normally get when you hatch Eggs.

This means that:

• your usual 1km Buddy Pokémon will give you candy every 0.25km
• your usual 3km Buddy Pokémon will give you candy every 0.75km
• your usual 5km Buddy Pokémon will give you candy every 1.25km

After November 1st, your Nidorina will begin finding you candy after every 3km again, so take advantage of this while you can!

Solution 2:

This is the Halloween Event

Halloween is here, and Pokémon GO has its own treats to give out! Starting October 26 and lasting through November 1, you can receive way more Candies than usual. Every Pokémon you catch will award six Candies—twice the normal amount. The Professor will give you two Candies instead of one for every Pokémon you transfer to him. And your Buddy Pokémon will award Candy four times faster than they usually do. Plus, you'll get twice the Candy you'd normally get when you hatch Eggs. Walk and catch all the Pokémon you can to get your fill of Candy during this special event!