Pawl/Ratchet mechanism is damaged. Take it back and get the wheel replaced, you generally have a year warranty.
Depending on the brand of hub, it might or might not be easy to take the freehub off and inspect the mechanism but given it's so young, it shouldn't be your problem, the shop should (in many places by law) repair or replace at no cost to you. Under 6 months in the UK, you can declare it's not fit for purpose and have an exchange; over 6 months, the manufacturer decides whether replacement or repair is better. The mechanism could be considered a wear part and you might have abused it but as so few wheels fail this way after such a short time, your case is strong.