Why do I have to pay CSA to mother when child lives with grandparents?

I need to know where I stand on this. I’ve been paying money for my child thought Csa and my child did Iive with her mum however she hasn’t done all year. She has been living with her grandparents so after talk to grandparents I said I would pay them so phoned Csa who are been checking up on the mother of my child to “prove” that my child isn’t with her (they can’t get hold of her due to mother being kicked out of house and no phone) so I said I would be stopping my money going to the mother of my child (due to her not having her) and pay grandparents insteady,how ever Csa phoned me today to say I had to pay the mum as she is getting child benefit so had to still get Csa money too. Is this right? I’ve blocked the money going but don’t want to get in trouble. I’m trying to do the right thing. Need help please as if mum gets this money AGAIN  it will be used for drugs etc…..