It's not a reflexive, but a pronominal. I have always seen "quejarse".
·¿Por qué te quejas, te duele algo?
Why are you moaning, are you in pain?
·Me voy a quejar ante las autoridades.
I'll complain to the authorities.
·No se quejen, de cualquier forma haremos el examen.
Don't grumble, we'll do that test anyway.
·¿Por qué se quejó el cliente?
Why did the customer complain?
·No quiero hablar con Juan; siempre se queja de todo.
I don't want to talk to Juan, he's always complaining about everything.
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